Lets start with his abilities:
Stealth - Pretty good on a warbeast, even a light as you aren't going to kill him with blast damage at ARM 15 (17 in most lists). Makes him survivable if he doesn't get thrown into combat however he does not have a ranged attack so if you are keeping him back he's not doing anything. You could sprint him but if you are paying 18 points for Rorsh, Brine and a Road Hog you probably have higher priority targets for the animus.
Man Eater - Free charge against living models. However without reach he is going to struggle to tear up units. MAT 5 hurts a bit too.
Beguile - his animus is a great late game piece (living non-caster models within 5" must advance towards the nearest model with this animus, costs 1). Once you have attritioned your opponent down and they don't have a lot left you can control the movement of some key infantry models or beasts. The short range on it and the lack of survivability of the beast when it gets stuck in means that you probably don't want to be using him early game. It may be cute to put a Janissa wall between him and some non-reach infantry but you really don't want to be wasting the wall on a 4 point light.
Paralysis - both his claws have paralysis, and it's straight paralysis, not crit paralysis either. Takes living models hit down to DEF 7 and stops them from running or charging.
Adds up to - If he's going to have a place it's going to be against in the hordes match up, Circle and Legion in particular (hordes is varied enough that you can't just take an anti-hordes caster). Paralysing an Angelius or a warpwolf will mean that troll beasts have no problem taking them down. And late game you have the ability to exercise some control over that one beast/unit that you are worried about. The stealth will let him survive to the late game but doing that means you are playing 4 points down for most of the game.
Lets go through lists:
Borka (Family Reunion) - His claws are not open fists so he's out.
Calandra - I think I found a home for him here. I love Burrowers with Calandra and she plays them very differently than other casters, she likes to keep them closer to the rest of her army rather than using them as an early rush. What that does is it opens up the possibility of troll gang (charge of the trolls) and having a cheap beast with paralysis to charge whatever the burrowers are after, kept alive with stealth... I think it works.
Captain Gunnbjorn - No idea. Never played this guy, don't intend to.
Grim Angus - No. Grim if I take him is my Cryx match up. Grim is pretty good against circle too (I did have that one epic game where I completely tabled a circle army in one turn) but he's usually bombing beasts to death and you don't want a crappy light engaging them when you do that.
Grissel1 - Another Cryx match up. Grissel wants troops so her warbeast points are very tight. However the extra melee troops does mean that a cheap charge of the trolls target would be nice, however unlike Calandra she needs infantry clearing which means that the thresher on the Axer or the aoe on the Pyre are also needed to contribute. Probably not in this list.
Grissel2 - Grissel2 is an underrated monster in the protectorate match up and provides a real problem for a lot of builds (inhospitable ground, lots of high ARM multi wound troops and great shooting). The night troll does absolutely nothing for her.
Doomshaper1 - Loves the hordes match up and you can fit a night troll in his theme force if you drop a Mauler down to a Pyre troll. I would not do this. He has plenty of AOEs and the Mauler fits better with the ARM skew and helps out against colossals.
Doomshaper2 - Again he can technically fit but you want to take a unit and all the dire trolls you can. The stuff he does is not needed in this list.
Hunters Grim - Maybe. Probably not. I have no where near enough experience with the caster to tell. He does seem like a good hordes match up because of Mortality but I'm not really seeing him wanting beguile or paralysis.
Jarl - I tend to run a lot of minions with Jarl which means he's battle group needs to be able to kill heavies and I want an Impaler for Jarl himself. Again it's a maybe, I need more games with Jarl too.
Madrak1 - He has no place in a stereotypical troll brick.
Madrak2 - Madrak2 lacks a hit buff and he has a lot of infantry. A bit of control would not go astray here. I'm sot quite sure how to fit him into the list but he is an option here. Beguile on Madrak himself seems like a strong move too given how damn near invincible that guy is.
