Im not sure you can finish the last 2 stages, the problem is that you have no source of dmg (no reflect, no strong stack possibility, no ult lockdown combos, no low/high end “event carry heros” such as Wilbur, Proteus, Wukong or others). So every fight will drag on for a long time, which means you will run out of items. And if you go full dmg items, then you are most likely not getting to the bosses with all 5 heroes alive in most trys, which means you will have to retry many times until you get a good opening that charges you a lot of mana.
As for Stage 7:
Belith, Kailani, Gadeirus, Gretel, Boldtusk
It will take a LONG time to kill Marie but as long as you keep dispelling, blocking and ghosting her you can get ahead.
The best advice I can give you right now, is to focus on leveling one hero at a time instead of spreading your feeders into 10 different heroes. You are slowing your progress at every aspect of the game by doing it like that. Even in war, you rather use x5 unleveled heroes of the strong color against the tank, rather than 5 heroes leveled at 1/4 or 2/4.
If you have to level a 5* hero, then you can make a point if it’s worth it to max the 5* or get two maxed 4* instead. But newer players should be focusing on 4s and 3s, sure it not fun ignoring the big powerfull 5* but you will get more fun out of the game by having a deeper and usable bench, than just running around with 1 big hero.