It looks like one of your screenshots may not have loaded, but from what I can see:
Proteus is one of the single most game-changing heroes for everywhere.
Being able to freeze bosses from gaining mana for 3 turns while tossing tiles at them to charge your specials and do damage is huge. Same goes for Raids and War.
I take him nearly everywhere — Raids, War, Season 2 Hard, even holy titans. One of the only places I leave him behind is the Season 2 Magic Night levels, where his effect is limited to 1 turn, and thus not super useful.
But he’d definitely be a very top priority to level.
He’s one of the only heroes in the game that I’d even make the case for power leveling using all feeders, and not just purple. His impact is just massive.
Proteus and Li Xiu are a great combo for the late Season 1 stages too — mana control goes a long way to beating tough bosses.
Apart from Proteus, I see 2xGrimm, who’s one of the best hitters in the game, particularly amongst blue, and well worth maxing one or both of. His defense debuff is huge for titans — once he’s ready, he should join all of your titan teams, except holy, where Cheshire can stand in.
I also see a second Li Xiu, who wouldn’t be as high a priority, but you’d get good use out of for War.
Duplicate Kashhrek aren’t likely to get a ton of use. While he’s fantastic as a tank on defense, he’s not as well suited to offense.
Skittles isn’t bad, and attack debuffing all enemies can be useful — but at Slow speed, the usefulness vs. Scarlett debuffing 2-3 enemies at fast speed will tend to mean Skittles sees little action.