Can't seem to do any progress on the second wall (fire wall) even with negative resistance. Am I missing something?
Plasma reaction and skill points, most likely. Second wall (current endgame) is a Cannons + Balloons spam. I went with 4 arrow, 2 ballistas, and all cannons after that on each row. Might be better to bring 6 arrows and 1 ballista on each row for more bio stacks. Damage goes to hundreds of millions in the end, and the second wall melts.
Points in: row 1, skill 2; row 2, skill 1; row 4, skill 3; row 5, skill 1. This is a minimalistic setup. If you have more points, take row 4 skill 2 and row 5 skill 2. Everything else doesn't matter.
After filling each row, buy 10 balloons, and after filling 3rd go all balloons.
In Alchemy, 1-2 level in Root, the rest spread between Molten, Burn, Plasma and Breaker. In early stages, priority on Molten to make economy going. And don't forget to upgrade Cannons to max. :)