I really, really like Wu Kong. I would build a team around him- Wu Kong, Rigard, Melendor, Gormek, Sonya
You are already leveling Wu Kong, Melendor and Sonya.
Rigard has one of the best heals in the game ( 42% all allies ) and the only 4* cleanse. I have one at 4* 4.70 and bringing up one to 4* 3.60 for war.
Gormek is part of the Ramming Pulverizer trio ( see notes ). Grimm has a better attack stat, and Boltusk is a better tank, but Sonya fills your first rainbow team blue hero slot, and Boldtusk’s heal is smaller than Rigard or Melendor. Boldtusk’s special doesn’t last as long a Ramming Pulverizer.
Sonya ( you are already leveling ) has a higher defense and HP than Caedmon, the other Piercing Strike sniper. It is useful to have a fast mana speed hero to balance the average mana speed heroes on the rest of the team. Grimm would be useful, but Gormek already fills your first rainbow team Ramming Pulverizer slot.
Wu Kong 4* 4.70 with two healers ( and one Ramming Pulverizer ) can take you all the way through Season 1, current Season 2 normal, all rare quests, all seasonal events up to 3* ascension items.
==Alternative==
You could swap Gormek for Kelile and Grimm for Sonya in the above recommendation. I find Kelile’s DOT to be less useful than Sonya’s debuff all enemies.
MY PERSONAL OPINION
Personally I would level up a rainbow 3* 3.50 hero team and a 4* 3.60 hero team first, since you can use double colors.
I have the following leveled which would make you a good rainbow 3* 3.50 team:
Belith ( medium heal all allies, several for double color 3* and war )
Valen ( sniper with defense debuff, several for war )
Hawkmoon ( medium heal all allies, several for double color 3* and war )
Bane ( sniper with blind, three for war )
Tyrum/ Balthazar ( sniper with debuff all enemies/ sniper with high special skill damage, several for war )
