Nice, another Killer to befriend in Monster Chopper. Mine is at level 59 right now and has over 1200 ATK IIRC. I dunno if I'll level him up much more. It needs more exp than my other QCK units (Sanji and Hina).
If you want to try your Killer you can send my a PM and I'll change my captain. Sasuke and I did some 0 stamina island runs with a double Killer combo and it does some really nice damage.
I got with a maxed orb over 170k damage with his Killer. Sadly, any enemy lasted enough to unleash a full combo
Leveling INT units should be "easy" since they get the 50% EXP bonus for matching color.
What kind of team are you using? Gear 3rd or Slasher?