I prioritized attack (taking him to +18) for three reasons:
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I have Mother North, which makes me significantly less troubled by a hero dying.
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When I use Grimm it is in a blue stack with some combination of Frida, Kiril, and Aegir. In those situations I want him primarily for his tile damage, which he contributes even when he’s dead.
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Here is the analysis I did on his attack, defense, and hit points (I will follow the convention of 2*Def + HP = survivability):
Base: 716 / 531 / 1132 = 2,194 survivability
Lvl 18 Atk: 827 / 585 / 1204 = 2,410 survivability
Lvl 18 def/hp: 752 / 621 /1312 = 2,554
Just for fun, let’s go level 20, maxing survivability: 752 / 636 / 1357 = 2,629
A couple of reference points
Average 4 star hero survivability (maxed, no emblems): 2,370
Average 5 star hero survivability (maxed, no emblems): 2,775
Attack Grimm barely has above average 4 star survivability. Even maxing that way is not that much better and I don’t think is worth giving up SEVENTY FIVE points of attack. He’s still substantially below average 5 star survivability (which feels like the relevant comparison to me since my raids are in the Diamond Arena). My conclusion was essentially that there’s just NOTHING you can do to keep him alive, past a certain point. Let him hit hard, move on.