*shrug* I don't know how possible it is, or feasble in your place...
Disclaimer: I am not always an asshole, but I have serious asshole qualities when I want them. This worked best when I was in the Navy and nobody was getting fired or moved around no matter what. It's quite simple, find out everything you can about the person, what pisses them off, what frustrates them, what makes their job harder.
Every so many days, pick something small on that list, do it. Don't be obvious until much later in the game.
Once a month, find something fairly big on that list, arrange for it to happen, or do it or whatever. Same rule on obviousness applies here.
Stay away from anything illegal or anything that could get you fired. The less witnesses the better, unless they share your dislike of the dickhead, in which case you have allies. Enlist their aid :) Get the asshole to quit.
Granted, when I was Active Duty, there was no quitting, so I figured if the guy was going to make my life hell just because he could (I worked for the assclown) I'd return the favor.
The coup de grace to this whole deal was the week before I started my 53 days of terminal leave (yay, 2 month paid vacation!) we had a division party at my LCDR's place. We were all drinkin, eatin pizza, havin BBQ. Good stuff, me, all the guys, and the assclown on the other side of the table. My LCDR was a knob, but overall, not a bad person and better than most officers) but he knew I was into guns and he was an amateur. So he says to me "Hey, ET2, you gotta see this!" and runs inside.
I'm curious at this point, cuz his tone of voice was like a kid in a candy store... not exactly something you see an officer direct at enlisted, at least in the Navy where we're stuck in the 50s and us enlisted types have all our own facilities. (Side note: I'm not really that bitter, but sometimes it sure felt that way.) Anyways, he comes back with a black pelican case and gently sets it on the table in front of me... I pop the latches, open the hatch (which hid the contents from anyone except those sitting right next to me) ... everyone's craning to see what it is, cuz my eyes light up.
I reach in, and heft out this ~26" HBAR Armalite AR-10 (Barrel length/weight is estimated, I'm no expert on AR-10s) and turn/shoulder it at the same time... the funny part is that I was turning out to the yard so I wouldn't be muzzle sweeping anyone after I'd already checked there was no mag and the bolt open, and the asshole... well, he squealed like a little girl and dove off his chair because he thought I was gonna shoot him. I laughed, the LCDR laughed, everyone except the asshole, and including my Chief laughed our asses off. Dude got up, bone white and someone asked him if he'd pissed himself.
Either way, I'd say that was the perfect cap to about 2 years of counter-asshole operations.
Your mission, should you chose to accept it...