Keep in mind that normally damage-focused classes, while being good during the stage they become unlocked, tend to fall off since they don’t provide any utility. Their skills won’t do as much damage as they do earlier, since monsters get more HP and def/res as the game progress.
Quick cast from C3 wizard is loved by R4 Elementalist and R7 Rune caster 
some said that it’s a must have even.
R4 Sapper C2 broom trap work great with R6 Rogue steal for double broom trap
R4 Quarrell Shooter C3 running shot make a whole new type of play style gatling gunner with R7 Musketeer XD
Those are some of the most popular combos with low rank classes
And ofcourse many other hidden combos people didn’t find out yet :3
wait a min, go back to the sapper + rogue part.
Sapper plant the broom trap, then use rogue to steal their own broom trap, but the broom trap that already planted still continue to deal damage while you get another one on your hand to use while the first one is still on cooldown
I think I watched a pvp video of a Rogue using Capture on his own Broom Trap and on a foe’s Flame Ground at the same time and then placing them both on the opposing team’s feet, that was the funniest.