You did the right thing by maxing some 3* first, and the one you maxed are pretty good !
Green : Finish F.Tuck, he is almost there. Then Mnesseus and pray for a green 4*, it would be nice for you
Red : Finish Azar and pray for other red, like Hawkmoon or a 4*
Purple : Get Rigard to 3/60. If you have the trap tools, max him, he is the best 4* healer!
Yellow : Finish Kailani, her skill is very useful in a lot of situation, like the final boss of season 1. Get Li Xiu to 3/60. Max Melia. Get Li Xiu to max if you have the mats.
Melia is very strong and useful and a 3/60 Li Xiu will do you good for a few time, no worry to max her right away even if you have thw mats. Only then, touch GanJu
Blue : Finish Karil. Max Valen. Get Miki to 2/60 (see notes). IF you have no other 4* blue, touch Agwe. Stop with Greymane, unless you have no other project, he is bad, at best
With Rigard and Li Xiu at 3/60, you will havea very good core to play along. The 3* you have a great and will depend on the situation.
Mnesseus and Belith for dispel will be very useful
Bane hits very hard and against boss, his blind is very good.
Valen and Ulmer defense down will make everyone on your team stronger and if you fight low * titan, they will help your team.
Melia special skill helps a lot with tile damage and can save your a$$ if it kicks in
Nashgar hits hard and is DoT is useful
Azar mana down is one of the most precious skillset in the game. Every special skill triggering mana or stopping special skill are wonder.
Also, i hope you are part of an helpful alliance to kill titan, take part in wars and have helpful tips from your alliance member. The alliance playing (titan, war) is a great way to acquire ascension materials. If you are not in an alliance, check the ingame chat or #alliance-recruitment
If you have other question, feel free to @ me and check the #player-guides, ad there is some very useful tips there
The Miki case
Although it is not advised to level a 5* before a 4*, your 4* is kinda bad and Miki is kinda a diamond to be polished. Is usefulness against titan will be huge and, again, will help a lot against titan. In other area, like PvP and PvMonsters, his effect won’t really be useful as the hp requirement can be a letdown. Anyway, you should give him some love if you don’t have the luxury of getting another 4* other than Agwe. Get him to 2/60 at first, and get to Agwe then.