Help with Erymanthos Fight

[ZombiePilot]

So for whatever reason when I'm fighting the big boar, I can't seem to hit him. Whenever Erymanthos has even a single stack of retaliation(stacking 20% damage reduction), my attacks seem to miss or hit but do no damage. What gives? What do I have to do to kill this fat ♥♥♥♥♥♥?

Edit: even when the boar doesn't have any retaliation I still sometimes can't hit him

[Spawnling]

Fire, Posion, Bleeding are the easiest ways. When they trigger they seem to ignore Thick Boar Skin (just tested it). If you have some multi hitter with Bleeding Oil the fight is rather easy.
- Burning (5% damage): Gets removed by his charge, so not that great.
- Poison (1% per stack): My Cutthroat applied 13 stacks per turn. It's easy to stack it higher.
- Bleeding (5%): If you have Bleeding Oil to trigger its damage multiple times per turn something like a level 12 Pugilist can kill it in a single turn.

Other Options:
- Critical Hits: If you have 100% crit chance most of your units will deal enough damage to ignore its defense.
- Fever: +10% damage per stack.
- Bow Herald of The Eye: Dedals Triple damage if the target has no guard or armor.

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Thick Boar Skin:
Damage taken is reduced to 0 if less than 8% of its maximum Health. At the end of the round, removes all applications of Poison, Bleeding and Burning. [Passive]

Wild Rage:
Each time this unit is hit by an attack, it gains one application of Retaliation. [Passive]
Retaliation: Reduces damage taken by 20% per application. [...]