Here's my take on Captain Gunnbjorn. He has to be one of the least loved and under-represented casters for our faction. Why? Well because he doesn't have a stupid overpowered theme list or any particular synergy with troll melee troops. That and his feat is somewhat on the weak side. So why do I take him? Well, for one he looks bad ass. Two, when people don't see something, they usually don't know how to counter it. He can use that to his advantage....some, a good player will know what his limitations are and push them. I think he works just fine. His ability to give out snipe to some pretty hearty ranged units (croak raiders) and his spell guided fire which gives boosted attack and damage rolls to his battle group, makes him straight deadly in a fire fight. His feat, like I mentioned, is not really the greatest in the world compared to the others. It's not bad either, just has to be used defensively. It gives friendly faction models cover, immunity to knockdown and blast damage. Used correctly and your guys will be +4 DEF and won't get knocked down if they have to make (and pass) a tough check. He really does have a hard time cracking heavy armor though. If you're facing down a gargantuan or a colossal or hell, anything from Khador, it's going to be an uphill battle. Also, since he doesn't really have much in the way of melee, when you get closed in on, it's pretty much done. Proper positioning and good use of snipe should make short work of anything that can really hurt you before they can touch you.
I posted this the other day, but here's the list I'm building toward:
Captain Gunnbjorn
Runebearer
Dozer and Smigg
Dire Troll Bomber
Troll Impaler
Dhunian Knot
2x Max Croak Raiders
2x Feral Geist
2x Northkin Fire Eaters
It's just a straight up shoot the shit out of people list. The Fire Eaters are there to help with stealth, the geists are there since I had extra points and didn't know what else to put in there. They may come in handy every know and then, but I don't expect a lot of return out of them. The croaks are monsters under snipe and can easily take an armor 18 heavy with one unit of them. ARM 20, gets a little sketchy though.