Trolls are possibly one of the worst used pieces in an Orc team. Many an Orc coach complains about the unreliable troll ruining a turn or failing that crucial block. This can be true, but used right they are a powerful piece that your defence can solidify around or as a distraction to keep opposing players tied up. An Orc team with four Black Orcs and a Troll is a formidably strong proposition. Khemri & Lizardmen can just about out strength it.
Their 4519 stat line really makes it clear they are blockers. However its the skills that make the difference. Mighty Blow is good, and means they can be effective bashers. Many get sucked into thinking this means you need to hit with them often. However Really Stupid & Loner make them unreliable. If you need to do anything with them they have a 1/6 chance of failing with a good chance of wasting a reroll. Worst of all Really Stupid means you need to keep another player adjacent to them or move in otherwise its a 50% chance of losing the action and tackle zones. Always Hungry and Throw Team Mate mean you've got the option of throwing goblins. I'll discuss this more later.
Regenerate is useful. Sometimes it will be a life saver - maybe literally! Generally speaking Orc teams don't get hurt too much, so when Trolls are hurt you have the option of using the apoth before rolling to regenerate. With a badly hurt or even miss next game I'll generally save the apoth and hope regen kicks in. Permanent injuries, excepting -AG and -MA, and deaths I'll use the apoth. Even if the apoth fails you've still got the regenerate as insurance. What regenerate means though is that when you've used your apoth you can still risk putting the troll in the way of nasty opponents in the knowledge even in the worst case you've a 50/50 chance of getting away with it.
So how to use Trolls? What you need to do is work with them so that Really Stupid minimises its impact on your game plan even when failing. Try to position players, and execute follow ups so that the Troll starts your turn next to a team mate. That way you won't need to waste a player moving next to the Troll, just to let the Troll move. Partly you can do this by "compressing" the play. On offence this is relatively easy - keep the Troll within or near the cage as you roll up the pitch. On defence it can be trickier when against flair teams who want to spread the play around more. Often then the best action for the Troll is none at all. A S5 player with tackle zones is a useful defensive piece. A stupid troll without zones often a gaping hole for the opposition to run through.
When taking actions with the troll the basic rule of thumb is when taking a Move action do so as soon as you can in the turn. That way if you do get a stupid Troll you can re-plan the other moves to compensate. When blocking, or taking any other risky action, do so as late as possible in the turn. Really Stupid failures can hurt, but a knocked down troll early in the turn can be fatal. 2 dice blocks with trolls will turn over 1/9 - even with a reroll you are willing to lose on a failed loner roll that's about 1/16. Even 3 dice blocks aren't that safe - being less reliable than 2 dice blocks with Block or Wrestle. So when you block always think that you've got about a 1/4 chance of failing stupid or the block. So block every turn and that's twice a half.
General advice for Orc teams is don't blitz with Trolls. Even a 1 dice block with a blitzer is more reliable.
So what should Trolls be used for? Trolls are great punch bags. Cheaper than any other Big Guy, S5 means even other S5 players needs assists to take them on, Av9 makes them hard to hurt, regen gives insurance.
Trolls are great as tieing up opposition players. Tie up a nasty S3 or S4 player (MB/Piling On) with a Troll and your opponent will be annoyed as hell. Against Claw players you are better off trying to keep them on the ground or restrict them to blitzes. Against S3 teams try to keep a couple of players in your tackle zones. They can commit more to trying to block you, risk 1 dice or 2 dice against blocks or just sit there. Anyway you are getting more of their players to be negated by one of your.
Trolls can bash - with the caveats above. Aim for 3 dice on weak targets to maximise the value of Mighty Blow. Generally speaking if I'm going to take on another Big Guy I'd rather have a comparably reliable BOB do it than a Troll.
Trolls can throw goblins. Doing this is very unpredictable. It will only work about 1/4 when you've factored in stupidity, hungriness, fumbles and landing rolls. However doing so when the goblin does not have the ball won't result in a turnover, so your downside of a failure is pretty low (normally the goblin getting hurt). The only time I'd throw with the ball is for the one turn attempt at the end of the half or a real desperation play. There is an argument that having the capacity to do so on an Orc team is worth having - even if you only get 1/6 successful attempts.
In terms of skill progression Trolls get even better at the punchbag/tie up roles with Guard and Stand Firm. Grab is a good 3rd skill. After that value drops off a bit. Thick Skull for some extra resilience, Break tackle helps you reposition yourself. Strong Arm helps when throwing goblins. Juggernaut helps blitzing - but you shouldn't be blitzing with troll. Piling On helps with cas, but then you are slow and need to stand up. Multiple Block looks good, but the unreliability increases significantly with it. Without block the two 2 dice blocks you'll have about a 1/6 chance of turning over (plus a 1/6 of failing stupid). I'd only go for Multiple Block if I got Block or very late.
Doubles are where it really kicks in. Block is an automatic first double because reliable hitting makes them much more valuable players. Pro if you are lucky enough for a 2nd double because it significantly increases reliability.
Stats - on a 6,4 I'd probably take Guard or Stand Firm ahead of the stat. After that it would depend on the league. Bash heavy and claw light +Av is very handy, other wise +MA is always good to have. +AG is a waste, don't take it.
+ST is a debatable option. S6 players are really quite frighteningly good. They can soak up other big guys more easily, get 3 dice more easily and are harder to move. However Block makes Trolls much much better. I think on balance Block is a slightly better option, but I prefer +ST because you might get another double and the psychological factor can be significant too.
Big & Stupid
Trolls are big and stupid, and if you leverage that correctly you'll get great value of out them. If you resist it and try and pretend they are something they aren't (bashers, blitzers) then you'll find them unreliable and probably be better off without them.
Orc teams with four BOBs operating around a Troll form a very strong a solid line that most other teams can't really compete with. That means you can direct the play as you want - on offence smashing the holes for the cage to advance through. On defence you can stop most other cages in their tracks, or against flair teams force them where you want and then box them in.