I beat Spicy mode and all I got was a T shirt unlock Hot difficulty. The Demo final boss was such a brutal fight but I suppose that was intentional to draw in roguelike lovers that enjoy a difficult challenge. Poor Possum got killed by a reinforcement archer (who entered at faster attack order) in one shot which crippled me at the start. I just tried to burst down the boss only for Shamans creating tons of portals and the boss becoming immune to dmg/status effects until they were all destroyed.
That gameover did let me meet God and learn who was granting boons to us. Though I was hoping he'd help me more than just a rewind time to the start of the game. Like a starting perk? Well, this is a roguelike so meta progression/permanent passive upgrades like getting a third Boon choice (instead of just two), one Boon reroll, or being able to lock an unpicked Boon (if you want both) so you can grab it next time, are probably already decided for the completed game.
If you haven't thought of it already, I think a Rat King boss would be a cool idea. It'd be a fleshy mass of rats attached to each other like some sort of eldritch, zombie, rat hydra.
The Promotion mechanic for the Rats was interesting (if annoying at times) and means you really need AOE/crowd control/high movement to stop another Rat from Promoting. In terms of Poison/Vampirism/Explosion/Haste/Shield/Split Dmg/Flat Dmg Reduction, the most troublesome Elite are definitely Shield, Haste, Vampirism, and Split Dmg if you don't have AOE. They let the rats live longer and Haste lets them steal trash faster (if trash is right next to their spawn/exit!).
Anyway, second run I did way better and didn't manage to lose a single piece of trash in any level. I got Turret II/Gravity II/Vampiric for Possum, Toxic Gas II/Shield OR Warp for Skunk, Rage II/Shockwave II for Badger, Bounty II/Vanish II/Backstab II for Raccoon.
I really feel like you need to get those specific Boons to win. Mainly Turret II/Toxic Gas II/Shield/Rage II/Shockwave II or Great Leap/Vanish II. I needed all the turret and gas dmg to kill enemies in Spicy mode. Rage II is important for Badger's dmg ceiling and Shield to let anyone in danger of being killed/tank the final boss. Vanish II lets Raccoon stealth 5 times as opposed to only 2 times. Since he's squishy melee unit this is integral to keeping him alive.
I gave Mana -1 cost item to Skunk. She needs it since she casts multiple high cost spells per round. Toxic Gas II is so satisfying to see enemies die when they start their turn/run through it. Way better than Fire/Ice/Lightning imo. Shield blocks 1 hit which is valuable as there's no healing or avoiding dmg otherwise besides Vampirism/Stoneskin (Badger)/Stealth (Raccoon)/Warp (Skunk). Also necessary to stall and survive the final boss. She needs 7 MP+ regen.
I gave two blocks of HP to Possum so she wouldn't get OHKO again. Two MP so she can spawn a third turret without having to wait a turn.
Turrets are huge force multipliers which is important to deal with the ridiculous amounts of reinforcements. Some stages the enemies get to move first and they start next to trash cans so you have only 1 round to kill them before they escape. So if you don't have high enough movement or a strong Ranger/Mage build, you are screwed. Especially if there's also a huge wave of enemies as well.
Raccoon just needs one MP regen to carefully maintain Vanish II as he relies on stealth to not take dmg. His duty isn't dmg but to pick off weak enemies like the rat archers (helped with Bounty II), destroy Portals, and block enemies from landing on his (invisible) space.
I gave the dmg reflection item + unused action storage items to Badger. She's the Berserker and her dmg comes from Rage II. She's meant to tank and store action pts until lowered HP and then burst with multi-attacks. While waiting, she Shockwaves to chunk and slow crowds at the start of battle. I give her Mobility so she can do hit-and-run Shockwaves and be a 2nd character besides Raccoon to destroy Hell portals. The passive that adds her MP to her max HP is useful but really you want Shockwave or Great Leap for AOE to deal with massive waves of enemies in later stages.