I assume you are part of an alliance and will write this aimed on improving your battles against titans and rare quests/events monsters (pve aspects). These aspects of the game are the best ones to focus on when you are still developing your hero collection, because these will provide you with the necessary ascension materials to upgrade your heroes.
Nevertheless it’s good you have kept all your 3* and 4* heroes, because you will need 6x5=30 heroes during alliance wars. Save your best 30 heroes.
Green: Caedmon is awesome and if you have the ascension materials it is a wise decision to ascend and level him up to 4/80 (4th ascension, level 80). If you don’t have those materials, I would work on Brienne next since you don’t have Wu Kong. Brienne her attack boost will give you higher titan scores. Berden is a fine decision after that.
Red: Gormek lacks attack stats, but just like Grimm (and Tiburtus / Wilbur; Atlantis hero) his special attack ‘‘Ramming Pulverizer’’ applies a defense debuff on your opponent. This causes your attacks to hit a lot harder. He’s also definitely worth the ascension materials. After that I would start working on Namahage or Nasghar (I don’t think you can choose wrong), unless you really like alliance wars then Hawkmoon will be the better choice. You’ll want at least 6 healers for that.
Blue: As for the same reasons above (red/Gormek) Grimm is worth the materials AND he hits way harder. Logical next step would be Triton who’s a sniper and increases your healing. After that Gato with his immunity buff would be a great choice, because you already have the defense debuffers Grimm and Gormek (Valen and Ulmer have the same special effect).
Purple: I would level up Sabina to 3/60 and leave her there. You will fully ascend Caedmon and like Sabina his special already dispells buffs from all enemies. Rigard probably is the strongest 4* healer and his ability to dispell status ailments from all allies is one you want to bring along in 90% of the cases. I would start on Rigard and finish him. After that, I would work on Merlin for diversity and survivability. He’s also nice for challenge events and raiding.
Yellow: Don’t touch Hu Tau anymore. For Challenge Events Li Xiu will be slightly better as his attack covers all bosses instead of Chao’s sniping ability. I would also favor Li Xiu for raid defence. Since Li is at 3/33, continue it. Then work on Chao. Once those two are done or you can’t ascend them any further yet, I would honestly start on Melia as her special attack is great for a 3* hero.
In general the team you mark as ‘raid defense team’ should have 1 hero of each color. This is to prevent your opponent from stacking multiple heroes of the same color against the color you have used more than once. That would give them the opportunity to take you down more easily.
Your raid defense isn’t so important yet, but you have the opportunity to make a challenging defense team for your current cup range. The centre hero (tank) should have a special which either harms your opponents quite a bit or which buys time for your other heroes to charge up their special attacks. This leaves you with Grimm, Li Xiu (and Gormek - but weak attack stats) as options for your tank. Since your strongest red hero is Gormek, I would leave out Grimm once Triton is equally as strong as Grimm. They have the same special attack. I would also place your average speed heroes on the flanks (next to tank) and the fast speed heroes on the wings (outer sides). Taking this in mind you have a few future possibilties:
Triton - Rigard - Gormek - Li Xiu - Caedmon
The AI fires from left to right. You want Triton to go off before Rigard to boost his healing.
Triton - Rigard - Li Xiu - Gormek - Caedmon
I would choose the 2nd line up, since purple doesn’t have hitters in the 4* hero range except Tiburtus.
Right now I would use this defense team:
Caedmon - Grimm - Gormek - Sabina - Li Xiu
Hope it helps. 
