I think you could manage to win if you bring
Along with mana potions, Dragon Attacks, Bombs and healing potions.
Don’t let the bosses charge or you are done tough…
You should arrive to the Boss Stage with your heroes fully charged, make mobs commit suicide by counterattack while keeping some diamonds on your board and then:
- Use Proteus on turn 1
- Use ony one Dragon Attack on turn 1
- Cast your specials on them on turn 1
- Fire the diamond on turn 1
Then you should aim to punch a hole into the bosses formation to gain more mana by sending your tiles trough the empty spot, try to keep your Bombs for when you would be able to kill at least one Boss (you can also use them if you need to lower bosses’ attack to buy time), don’t forget to use the Dragon Attacks every time their damage over time disappears…
And charge Proteus with mana potions if needed, your low-leveled heroes can’t endure the bosses specials. At your actual heroes’ level you don’t have room for errors. I wish you to succeed!
(ps last stage doesn’t have Green mobs but be wary of The Hatter, don’t kill your Gadeirus on him and if you have a better healer then bring him instead).