Hmmm . . . Well, I'm pretty lazy. So at first I'd probably do rather less with it than Jee-Han. But when I found out that there were nasty people who would be coming along with the intention of turning me into a mana-battery in a cage . . . yeah, I'd do some grinding, some thinking and some experimenting.
On the basics: As a mostly-solo guy, and on an instinct basis, while I might push INT harder than other stats, I think I'd insist on getting to a floor of 20 for everything. Partly to find out what it did, partly to avoid weaknesses that might be exploited. I'd train some direct physical combat; I'd emphasize sword-and-shield because I consider that frankly the most effective hand-to-hand weapon system, and unarmed martial arts because you might not have the chance to get out your weapons but you always have hands, feet and whatnot. I'd include some grappling, joint-locking and such partly because one on one they can be very effective and partly for a reason I will get to.
I would no doubt work on some basic ranged stuff along the lines of Jee-Han's.
I would work hard on stealth, and try to come up with stealth magics, like invisibility, shielding my mana use and so on. Luckily zombies are probably pretty easy to hide from, so it'd be fairly easy to start the skill going.
I would work on my instant-dungeon skill. It looks like if you know what you're doing, you can stop people from escaping, stack the odds against them, improve your training, stretch time and stuff. Also, if your skill is high hopefully you might be able to escape from an opponent's higher level dungeon that would otherwise trap you, negate the modifiers they try to impose, perhaps take the damn thing over and turn it against them. Controlling the setting strikes me as a big deal. I'd much rather be the one in control of the territory than have my opponent control it. As a natural ability user, I suspect my capacity to do that could potentially be stronger than some opponents otherwise tougher than me.
So if I'm stealthy the next step is damaging or incapacitating magics that don't obviously come from me. If I can kill while I can't be found, I can defeat people who should be far tougher than I am. This is a deadly combination. I think it's rare in online games precisely because it is so broken, but Jee-Han's abilities seem very open-ended. And I tell you, in tabletop RPGs this is one of the most vicious approaches there is.
Another thing to try for is mind magics. One major problem with the Abyss is, everyone seems to know more about what's going on than I do. Mind reading would be an excellent way to start levelling that playing field. It also has its tactical uses--hard to surprise you if you know what they're thinking. Once you've got some mind magics, more offensive variations might be possible--stunning, fear, mind control, editing what they see--lots of fun stuff in mind effects.
Jee-Han is right to think about mobility. Flight, teleportation . . . excellent ways to escape, get in position to attack, hold distance from infighters.
Missile deflection! Apparently there are ability users out there using GUNS. Others have bows, still others likely have magical missiles of various sorts, not to mention thrown weapons (axes, shuriken, whatever). If I could come up with something that, rather than putting up a head-to-head "Is my damage shield as badass as your attack?" shield, instead eased that stuff aside so it missed me, I could again potentially neutralize the weaponry of people basically more potent than myself.
Finally a couple of less fundamental, more exotic amusements that I might try to explore: Draining attacks and a damage shield. Draining attacks, more specifically transfers, where you drain from your opponent and gain yourself, seem to be possible; Jee-Han learned one from that zombie skill book. But hit points is not the only possibility. I would have mucked with that one and then tried to come up with other variants. I had a character once who had a speed drain; it was highly effective. As he scored hits, his opponents would get slower and he would get faster, until eventually they were near statues and he was the freakin' Flash. Then he'd beat them to a pulp. Lots of fun!
And OK, Jee-Han has this force shield thing that blocks damage. But it's made of the same mana-stuff as his arrows and such, right? OK, so how about tuning it a bit so that if opponents touch it, say by trying to hit or grab him, the shield hurts them? Now combine that with martial arts grappling attacks. Put a lock on someone and hang on until they fry.
I'd probably fiddle with crafting rather more than he has been.