333 speed, will go first nearly against everything since majority s/s breeds are 325. Often those pets rely on that speed to actually use their skills, so from get go Imp hardcounters them.
Nether Gate, if you're faster, it's free powerful attack without retaliation each 5 rounds, and breaks any combo enemy tries to set up.
Immolation Aura - works from backline, and Imp is a humanoid, so if you use that after getting hit and switch him to backline, you're looking at something like 500+ damage dealt and 500+ healing on Imp.
Oh, and he is strong against mechs, which are strong against Direhorns, which makes it even better in meta imo as anti-counter to god-pet (unless you just run 3 Direhorns).