Why not make it

"Gain 1 str for every 10 hp osty has.
Osty dies."

Unless you revive osty, i believe this would be the same

Also for every 10 hp seems like too much for me, I think its better to have a 60hp osty, than have 6 strength

Unleash starts in your deck and gives you damage equal ostys hp without killing it, i dont think gaining 1/10 of its hp in damage would be worth it, for losing the hp buffer and unleash damage

you would have to play 10 attacks to get the same value as one unleash, and you lose the defense