I surprisingly managed to beat the fire mode on my second try (first try I lost on the first round because I went against the barrier boss and used all my cooldowns are bubbles).
In order to beat it I would recommend leaving the fruits on the ground (don't collect them) as this effectively tanks an attack from each of the minions, you NEED the turret upgrade for the archer as you are gonna need them to both tank hits so your squishy units don't die to random archer shots as well as deal damage, your badger is gonna need the block II ability as it is the only way for them to stay in the fight when their health is low (which is the time when they are most useful!!!) and finally you rogue is going to need both the swap/dash and vanish II upgrades as he is the one who is going to need to zip around the outskirts and destroy all the portals and the only way he can survive that long outside of the center area is if he is always untargetable.
As for other optional things that I think are nice but you can work around not having: the great leap ability of the badger was an absolute godsend for wave clearing and just damage in general as he was dealing like 60 damage with an AOE spell by the end of the run which was the only way of one shotting the minions that got too close to the boss. As for the mage I think the swap ability is the most powerful upgrade in the game followed by the singularity upgrade as they just allow you to reposition both herself and the enemy minions wherever you want. The main thing I did with her was have my badger great leap into the center of all the enemies, have the skunk teleport him out, singularity all those weakened enemies into a group, and then finally plop down a toxic cloud on them, this 9/10 times killed most units that weren't the boss. The rest of the skunks spells seemed underwhelming to me but definitely upgrade the toxic gas II as well as the spell range if you can as this just allows you to position her more safely while still providing value. Finally, I liked having the archer have the poison upgrade for her arrows, that extra 3-5 damage was integral in avoiding have to cleanup enemies that were left with 2 or 3 hp and having it stack-up on the boss mede her damage go through the roof (especially when the turrets also applied it).
