This thread has a lot of really useful info about troops. I started it when I was a newbie last year:
Some quick answers:
- keep a couple of 2* and 3* troops of each color, even as you progress on. These will be useful in competing in the monthly Challenge Events. Also, as you gain hero depth, you may want to bring 3, 4 or even 5 heroes of one color on a team, and some extra troops will really help.
- long-term focus should be on 4* troops. These only come for the Epic Troop Summons.
- among 3* troops, I strongly prefer the attack-heavy ones.
- hang back on advancing 2* troops. You will get 3* troops fairly quickly.
- ideally hand back on advancing 3* troops if you are in a financial position to buy some troop summons. Feeders are scarce.