The Best Strategy to Build Isaac for Osh in Hero Wars: What to Upgrade and What to Avoid

Published: October 2026 | By: Trade Boy

Welcome, Heroes! It's Trade Boy.

Isaac is one of the most valuable heroes for achieving high scores in Osh battles, especially when building a team around Cornelius. However, investing in Isaac for Osh is more complicated than simply upgrading him to the maximum level.

Every upgrade matters, and a single mistake can negatively affect your entire strategy. Some upgrades cannot be reversed, meaning that once you invest in the wrong stat, you may not be able to fix it later.

That's why I'm sharing this complete Isaac upgrade guide, covering everything from his ideal stats to the skills, skins, glyphs, artifacts, Ascension, and Tree of Wisdom upgrades you should prioritize or avoid. Looking for the full teams? See the best teams for Osh.

1. The Four Most Important Rules for Building Isaac for Osh

Before spending any resources on Isaac, there are four key points you need to understand:

• First Artifact — Lil' Tes'Lin: Keep it at level 114.
• Health: Prioritize higher Health to help Isaac generate Charge more efficiently.
• Magic Defense: Keep it as low as possible.
• Armor Penetration: Avoid investing too heavily in this stat.

Let's look at why each of these points is so important.

2. Why Must Isaac's First Artifact Be Level 114?

Isaac's first artifact, Lil' Tes'Lin, is one of the most important parts of this strategy.

When Isaac activates his ultimate while his Defense Spheres are active, his first artifact provides an Armor Penetration bonus to the entire team. At level 114, the artifact grants 40,038 Armor Penetration when its bonus activates.

If Isaac activates his ultimate four times while the Defense Spheres are active, the team can receive the following total bonus:

This is a significant bonus for your entire team and can make a substantial difference in your Osh battle performance.

However, there's an important detail you must understand.

If you upgrade Lil' Tes'Lin to level 130, Isaac can destroy the Defense Spheres in just three ultimate activations under the conditions of this strategy. As a result, you may lose the opportunity to activate the artifact bonus a fourth time.

That is why level 114 is the recommended stopping point for Isaac's first artifact in this particular Osh build.

The goal isn't simply to maximize the artifact's level. It's to control Isaac's damage output so that your team can benefit from as many artifact activations as possible.

3. Why Isaac Needs Higher Health

Unlike most heroes, Isaac does not rely on a conventional Energy mechanic to activate his ultimate. Instead, he uses a special mechanic called Charge.

Isaac generates Charge by receiving Magic damage. Physical damage and Pure damage do not generate Charge through this mechanic.

His second skill also makes his Health an important part of this strategy. Higher Health helps Isaac accumulate more Charge when he receives Magic damage, allowing him to reach his ultimate more efficiently.

This is particularly important in Osh battles, where you want Isaac to activate his ultimate multiple times while the Defense Spheres remain active.

The goal is to give Isaac enough Health to generate Charge efficiently without compromising the rest of the build.

For this reason, Health is one of the most important stats to prioritize when preparing Isaac for Osh.

4. Why Isaac Needs the Lowest Possible Magic Defense

This point is closely connected to Isaac's Charge mechanic.

Since Isaac generates Charge by receiving Magic damage, increasing his Magic Defense can work against your strategy.

Higher Magic Defense reduces the Magic damage he receives. Less incoming Magic damage means less Charge generation, which can delay his ultimate.

If Isaac cannot generate enough Charge in time, he may fail to activate his ultimate four times while the Defense Spheres are active.

That would mean losing one of the most valuable artifact bonuses available to your team.

For a Cornelius team designed specifically for Osh, keeping Isaac's Magic Defense low is essential to maintaining the intended ultimate timing.

This is why you should avoid upgrades that unnecessarily increase his Magic Defense.

5. Why You Should Keep Isaac's Armor Penetration Low

Armor Penetration is another stat you need to control carefully.

At first glance, increasing Isaac's Armor Penetration might seem like a good idea because it improves his ability to deal physical damage. However, this particular Osh strategy depends on controlling how quickly he destroys the Defense Spheres.

If Isaac has too much Armor Penetration, he may destroy the Defense Spheres after only three ultimate activations instead of four.

When that happens, your team can miss the fourth artifact bonus, reducing the total Armor Penetration provided to the team.

In other words, upgrading Isaac too aggressively can actually hurt your overall strategy.

The objective is not to make Isaac as powerful as possible. It's to build him specifically for the timing required in Osh battles.

With these four principles in mind, let's move on to the upgrades you should prioritize and the ones you should avoid.

6. Isaac's Skills: Upgrade Them to Maximum

Let's start with Isaac's skills.

Unlike some of his other upgrade options, his skills don't require any special restrictions for this build.

You can safely upgrade all of Isaac's skills to the maximum available level.

There is no need to leave any of his skills underdeveloped for this strategy.

7. Isaac's Skins: Which Ones Should You Upgrade?

Isaac currently has four skins to consider for this build. However, you should be selective about which ones you invest in.

The Default Skin provides Agility, while the Masquerade Skin increases Physical Attack. These are the two skins you can upgrade for this build.

However, I recommend avoiding the Lunar Skin and Spring Skin.

The Lunar Skin increases Magic Defense, which reduces the Magic damage Isaac receives and can interfere with his Charge generation.

The Spring Skin increases Armor Penetration, which can cause Isaac to destroy the Defense Spheres too quickly.

Remember, this build is designed around specific combat timing. You don't want to sacrifice that timing just to increase Isaac's stats.

8. Isaac's Glyphs: Choose Your Upgrades Carefully

Isaac has five glyphs that provide different stat bonuses:

• Physical Attack
• Health
• Armor
• Magic Defense
• Agility

For this Osh-focused build, the rule is simple: avoid upgrading Magic Defense glyph.

Health is especially important because of Isaac's Charge mechanic. Physical Attack and Armor can also be upgraded according to your build, provided you maintain the required combat behavior.

Be careful not to invest in the stats that can interfere with Isaac's ultimate timing.

9. Gift of the Elements: Maximize It

You can safely upgrade Isaac's Gift of the Elements to the maximum level.

Unlike the specific stat investments discussed above, this is not an upgrade you need to restrict for the purpose of this build.

You can invest in it without deliberately leaving it underdeveloped.

10. Artifacts: The Correct Upgrade Levels

Isaac's artifacts require special attention because the first artifact directly affects the core strategy.

First Artifact: Lil' Tes'Lin

Recommended level: 114

This is the most important artifact restriction in the entire build. Keep it at level 114 to preserve the intended Defense Sphere timing and maximize the number of artifact bonus activations.

Second Artifact

Keep this artifact as low as possible.

For example, I have left my second artifact at level 10 and one star.

The goal is to avoid unnecessary investment when building Isaac specifically for Osh.

Third Artifact

The third artifact is less important for this strategy, but I still recommend keeping it below level 100.

Personally, I suggest keeping it at approximately level 80–99, with four or five stars, depending on your current progression and resources.

The important thing is to avoid upgrading it without considering how the resulting stats fit into your overall build.

11. Ascension: Stop at Level 3

For this strategy, I recommend keeping Isaac's Ascension at level 3.

Avoid taking it any higher if your goal is to preserve the intended Osh build.

As with the other restricted upgrades, the objective is to avoid unnecessary stat increases that could interfere with Isaac's required performance.

12. Tree of Wisdom: Upgrade It to Maximum

Finally, let's talk about the Tree of Wisdom.

Unlike Isaac's individual upgrades, the Tree of Wisdom provides bonuses that benefit all heroes associated with the relevant branch.

For this reason, I recommend maximizing the Tree of Wisdom rather than leaving it underdeveloped.

It provides broader value beyond Isaac alone, making it a worthwhile investment for your overall account progression.

13. My Isaac's Stats: The Build I'm Using

Now that we've covered the upgrade strategy, let's take a look at my own Isaac.

I've built him specifically with Osh battles in mind, following the principles explained throughout this guide.

Here are my current stats:

These stats reflect the balance I'm aiming for with this build: high Health, low Magic Defense, and controlled Armor Penetration.

Keep in mind that the exact numbers are not necessarily the only way to build Isaac for Osh. Your results will also depend on your team composition, artifact setup, and other account-specific factors.

The important part is understanding why these stats matter and how each upgrade affects Isaac's performance.

Final Thoughts

Isaac can be one of the most valuable heroes for maximizing your score against Osh, but building him correctly requires careful planning.

Before investing your resources, remember these key rules:

• Keep Lil' Tes'Lin at level 114.
• Prioritize higher Health to support Charge generation.
• Keep Magic Defense as low as possible.
• Avoid excessive Armor Penetration.
• Upgrade the Default and Masquerade skins, but avoid the Lunar and Spring skins.
• Do not upgrade the Magic Defense glyph.
• Keep the second artifact as low as possible and the third artifact below level 100.
• Stop Ascension at level 3.
• Maximize skills, Gift of the Elements, and the Tree of Wisdom.

Most importantly, don't blindly maximize every stat on Isaac. An upgrade that looks beneficial in a general-purpose build may be counterproductive when your goal is to achieve the highest possible score in Osh battles.

Plan your upgrades carefully, understand the purpose behind each restriction, and build Isaac around the strategy your team actually needs.

That's all for this guide. I hope it helps you avoid costly mistakes and get the most out of Isaac in your Osh battles. Ready to put him to work? Check out the best Osh teams with Isaac and Cornelius.

Thanks for reading, and I'll see you in the next guide!

— Trade Boy