you also need lobster to gain 1 str per thorn for those boss, game is very rng heavy

I think this is mainly due to a hefty chunk of missing content we don't have in the demo that should be in the full game. We don't have a bunch of synergies or other attacks that enemies can use. Right now, the only build that is guaranteed to work is basically scaling strength with stuff like flexes/cardio/power heal. Especially since the final boss of the demo can counter almost any other build. Even thorns doesn't work well unless you have hundreds of points.

Yes, building thorns does seem to be somewhat of a gamble early-game. It might be worth it for the other fights (especially Lizardberg), but then you have to hope you'll get enough recovery cards to help you get past Pop Cat like you said.

As for The Stone (Rock): getting ice cube attacks or cards that result in ice cubes is my strategy there since it slows down his flexes. Since his ultimate damage increases continuously throughout the battle, recovery cards will only do so much. You either have to drain his health fast or slow his flexes down so your recovery will keep you going while you chip away at him.