That's not a so simple strategy. Had to do it about 10 times to make it work, and succedeed only 'cos I've been lucky with the crits. If the Cinderweb is swapped out, just restart the fight. And I suggest to use Deep freeze on Kraken tentacle, if you're lucky enough to bring your Tiny Snowman till there. Lot of RNG on this strategy, maybe it should be marked out.
An Aldrussian Sproutling (121) actually worked really well for me instead of the Cinderweb (which I didn't have leveled). Keep Photosynthesis(3) up, Plant(2) yourself early, and keep hitting Leonard with Scratch. If it looks like you are going to die on your next turn (which happened to me when Leonard was low) just hit Plant(2) again for a massive heal, and then just replant yourself. Rinse & repeat against the Corrupted Slime. I managed to start fighting the Reanimated Kraken Tentacle with about 70% health left on the Sproutling, did some damage, and my Fel Flame easily finished the fight.