We have a troll bio-adept that is like this too, exploding people in one hit with a base unarmed damage of 19P (or 20P if you let knucks stack with bone density aug / critical strike), and very high defense/armor. Add in throwing knives, and he has a potent ranged attack as well.
As others have indicated, your best bet is to make peace with the reality of it (let him be the combat monster he is), and just make the runs varied and fun for all the PCs. Ratcheting up foes on a regular basis (simply to be on par with the troll) will not be fun in the long run, and it will likely kill everyone.
That being said, beyond the above, below are some things you can sprinkle in there (and I do mean, sprinkle - certainly not often).
Take advantage of limits on movement, and have a fight where enemies are all spread out. Presuming your troll has an AGI of 4 (since you said his STR was the max attribute), his walk rate is 8m and run rate is 16m. That's max movement for the entire Combat Turn (not initiative pass). So if troll has to run most of that in the first pass to get into melee with a foe, he won't have enough movement left to run over to the next foe in the second pass.
Take advantage of scenarios where outright killing people is a very bad option, forcing the troll to use non-lethal methods.
Use minions. Add one or two grunts. They're not there to amp up the NPCs, they are there to use up the troll's actions punching them (rather than the more important NPCs). Gives you maybe 1 or 2 passes more in combat.
Focus fire on the troll (within the same pass) will reduce his defense dice pool for each additional foe attacking him. Eventually he'll get hit.
Have an enemy mage use the Control Thoughts spell on your PC troll. This might encourage teamwork (mages must use counterspelling or their own Control Thoughts on the troll) and change the player's tactics in the future (as the prospect of their troll monster turning against them should sufficiently scare them).
Have an enemy mage apply penalties to the troll, with things like the Bugs spell (potential for losing an initiative pass), Chaos spell (potential for penalties to all dice pools), or Decrease Attribute [Agility] spell (potential to floor his AGI to make him less likely to hit, or even paralyze him).
Snipers + Ambush. This is a decent chance to "surprise" the troll (denying him his defense against the attack). The sniper rifles have base damage of 11P to 14P, and armor pen of -3 to -5. Add APDS ammunition, and it's even more deadly (troll loses more damage soak dice). Without the troll's defense test, you'll probably looking at bumping that up with at least 3 or so successes on the attack test. Even with the cheap Ruger 100, odds are the troll will take some damage and have to deal with wound penalties.
Enemy's mage casts Impvd.Invisibility spell on all his buddies (sustaining all those, being his only real function in the fight). Now troll must deal with the fact that he can't see his targets.
Grenades. Not only do they disallow defense tests, but high explosive ones start at 16P (with -2 armor pen).
Area spells. An enemy mage's Force 6 Fireball disallows defense tests (like grenades), has a base of 6P damage (and the potential for more), as well as a huge -6 armor pen. I'd imagine 1 or 2 boxes of damage might make it through. For more challenge, enemy mage now overcasts (say, Force 12), uses Edge/Reagents to limit hits to 6 [so he doesn't kill himself], and uses "aid sorcery" from a bound spirit during casting (to make sure he gets all 6 countable hits). Now you're looking at no defense test for troll, 15P damage, and -12 to troll's armor pool. This will hurt.
Presuming your troll doesn't have a weapon foci or the killing hands adept power, enemy spirits with enough hardened armor could be a problem for him.
The nemesis. A nice one-time thing, where someone your team has pissed off sends an equally powerful shadowrun team after them (with their own troll adept). Or just an underground boxing ring, where the champion challenges your troll to a cage-fight.
Trap the troll in a dome-shaped Physical Barrier spell. Now he has to spend actions punching through the barrier.
Distract the troll with an illusion of fake enemies (Trid Phantasm spell). Now he might spend actions punching the air.
Levitate spell on troll. Now he's floating upside down against his will, unable to run around punching people.