Khagan is of no use?

3* ascension items

I strongly recommend leveling 3x to 4x rainbow 4* 3.60 team before using any 3* ascension items, but it depends on your roster.

Red ranger

I am leveling Khagan to 5* 2.60 specifically for Trials of Serenity ( green / blue mobs with yellow / green / blue bosses ) and Trials of Nature ( green / blue mobs with yellow / green / green bosses ).

Khagan does strong damage against green mobs and bosses even while dead ( grave damage ). Even his weak damage against blue mobs is better than the 1 point of damage for a missing red hero. The only other Red Rangers are 3* Squire Rabbit ( Spring ) and 5* Red Hood ( Challenge event ).

I also find slow heroes less of a problem for Class quests, because many accounts cannot field a rainbow 4* 3.60 / 5* 2.60 team of good heroes for each class quest. So even fast heroes can take a while to charge if you are forced to run a team with 3x yellow 4* Chao or 3x green 3* Berden.

Scarlett and Gormek, together they fight crime

Scarlett and Gormek are very good red 4* heroes, though players who buy IAP often replace Gormek with Wilbur ( see Wilbur below )

( Two Rogues or comparing Guardian Jackal and Scarlett ( with a mention of Grimm ) )

( Ramming Pulverizer-Who's your favorite? - #13 by Gryphonknight )

Red 4* roster example
( Gryph, Doc - Can I get your advice? - #12 by Gryphonknight )

Boldtusk and Wilbur, together they fight crime

Boldtusk, especially at 4*+1, and Wilbur are very good heroes

Fighter Boldtusk
( Boldtusk or lancelot- for fighter class? - #5 by Gryphonknight )

Poor Gormek
( Gryph, Doc - Can I get your advice? - #13 by Gryphonknight )