As Rohn said, do some 3* rainbow teams first, I’d say, at least 2, good 3* you can find here:
(for red, Bauchan could be added)
Four stars then:
- red: Boldtusk, then Gormek for def down
- yellow: Li Xiu or Chao, Li Xiu could be the better one with her mana cut on all, Chao is an average sniper with mana cut - if you like Titans, then Wu Kong
- green: Kashhrek could be your tank (center) in defense, he’s good there, then Melendor as another good healer, maybe LJ then, Skittles is a bit mehh
- blue: Kiril, very good buffer and healer, I have 3 of him and use them regularly, nice additions could be Grimm, Sonya or Boril
- purple: whatever comes along, maybe start with Merlin to 2/70, good 4* would be Rigard, Tiburtus, Sabina, maybe Cyprian
When this is done, I’d focus on the 5* - it’s easier to get the ascension materials, if you have some good 4* as your backbone, with them you can do almost everything.
I’d recommend leveling 5 heroes at a time simultaneously, 1 of each color and feed them only 1* and 2* of their color, so you get an 20% XP bonus per feeder.