I should have read this earlier. But your team is good enough to win the last stage. I am used to bring 3* heroes to the final boss alive.
Yes 3* are squishy but if you know how to use your item, you can keep them alive. I have done that many times without healer (paladin + ranger, I even complete the first one without fully ascended 4*). I even bring 4 3* and 1 2* alive to win a last stage.
The items you shall bring is 10 Healing Potions, 5 Axe Attack, 5 Bomb Attacks, 5 Dragon Attacks.
Use 1 axe on the first wave
Ghosting between waves.
Use 1 more axe on the fourth wave.
Use all remaining bombing items on bosses
Use Proteus skill and in 3 turns you will win.
With this strategy, ever since I hit level 30, I always complete the last stage of trials, even with team power below 2300.