Troll stuff.
First off, a caveat from an old GM to a new GM. It's okay to consider what your options are to deal with powerful characters, but be sure to keep in mind that your players are the heroes. Let them be heroic.
Yes, they shouldn't have it too easy, but at the end of the day (in most games) the PCs should come off as the heroes, and it's
okay for heroes to kick a little mook behind. Heck, it's okay for them to kick a LOT of mook behind.
If they have done their homework and have a genuinely tough character...allow that character to be tough. He's a bloody Troll in 21/17 armor - he has the right to be a little cocky. Heck, he's paid a price for that right...Troll doesn't come cheap, and neither did that armor. Your average thug should be quaking in their booties.
With that out of the way - on to your question. How do you actually put some threat toward something that durable?
20ish armor is doable in a couple of ways. If your Troll was clever, he may have done this in a way that he basically normal looking to most observers and can move around in polite society. If he was a little less clever, he might be wearing Full Body Armor, or something of the sort - this is NOT acceptable in polite company.
If he's walking around SWAT/Military armor, that stuff is not legal. If he's starting with it, he needs the Restricted Gear Quality, and a license. If it's Forbidden availability, like Military armor, he can't get a license. Even if he has the Restricted Gear Quality, if Knight Errant sees him walking down the street with that stuff on, they'll react accordingly.
(Accordingly, in this case, means falling back and calling in a special team with Stun weapons (Electrical damage is good vs armor and low Willpower individuals) and Anti-Vehicle weapons. Explosives. Big nasty sniper rifles with armor defeating ammo. Oh, and definitely a Mage with Stun Bolt.)
I'm not saying kill him (though the Player doesn't need to know that) but you can make it clear that he can't goto the Stuffer Shack in that gear.
Now, if you Troll is clever and is wearing socially acceptable armor - well, he deserves a cookie. A cookie in the form of his stuff is legal, so he doesn't get hassled. As a big tough well-armored Troll, he can basically ignore most small arms.
Does this mean that he's going to walk through most low level threats unscathed. Yes. Yes, it does.
But the heavy hitters, the Bad Guys, the named villians (rather than faceless henchmen)...chances are they have taken the time, themselves, to get good armor. And maybe their OWN Troll. And they've definitely taken some time to to think about what they would do when a Big Bad Troll comes knockin' - which is probably hire themselves/learn to be a Mage with the right spells to go up against his dump stats, AP weapons, or enough expendables with high enough rate of fire to wear even a big bad Troll out.
In SR4 it is definitely possible to make a very very durable PC. The system is designed, in my opinion, to make PCs highly survivable - but they are not invulnerable. As a GM, your goal isn't really to KILL your PCs...it's to SCARE your PCs.
Really, at the end of the day, you want your PCs striding out of the smoke and flames, bloodied...but triumphant.
(Well, unless they do something really stupid - then, kick 'em where it hurts!

)
*Grin*
-Jn-
Ifriti Sophist