Jarl is one of my favouritive casters. First thing, you should play your Brick-list with him, like you used to do with trolls. He is more about fast striking and killing of usefull solos/UA of your enemy. So here are some tipps for how to use him.

Jarl got some nice stats and abilities. With speed 6, pathfinder and evasive nothing can stop him really and you need to keep him mobile. Swift Hunter helps him getting his pistols more in range of enemy models. with DEF 15 he is one of the more “evasive” warlocks trollbloods have to offer. Put him in a forest and Quicken/Weald Secrets on him and he gets DEF 19! And last he’s thought, so 1/3 chance that he suvives that killing blow.
Jarl comes with 3 Upkeep-Spells. Most people want all of them up at the same time, but mostly you dont need to.
I use “Quicken” mostly on Champions. Have you ever seen a champion-unit running 14 ” accross the board at first turn? Neither did your enemy. And with DEF 14, ARM 18 and 8 hitpoints per model your enemy will have hard times killing them. Sometimes you can use it on Jarl, to keep him mobile with 8″ movement and 12″ shooting. You can get some nice first kills on first turn, if your enemy begins the game.
“Tactical Supremacy” is good to repositon yourself. If doesn’t have “Quicken” on him, then I take “Tactical Supremacy”. You get 3″ extra for your next turn. Also you can use it lategame on the champions, to reposition them after a charge (to get arm 18 again). it’s also usefull on sluggers or other shooting units, just to keep them save.
“Weald Secrets” is also nice to have, but most of the times, I use a fellcaller hero. If you don’t have him in your list or he died in game, then you can use “Weald Secrets” to get your guys moving through that woods. If not, I would bother with this spell.
“Magic Bullet” is where Jarl really shines. Eventhough some pesky Cygnarian warcaster know this spell too, he never used it. But for Jarl its a good way to kill that solo, that help your enemy more then you. And the “magic bullet” doesn’t taget an enemymodel, you “choose it from the last model hit within 4″ of it”. This can make a hughe difference for abilitys like stealth or eSkarres feat. It is just an unboostable POW 12 but you can kill some small stuff. But remember, you need to hit a model first to trigger “Magic Bullet”. If you miss so does the “Magic Bullet” and its wasted!

His feat is defensive. It’s good, but you need to know when to “pop” it. Most of the times you should be using it, if your enemy is in charge-range. With those clouds, most enemy can’t look though then and therefore cant charge you or shoot at you. What I do, I divide my force. An hardhitting fast force and the Mainforce with Jarl and his beast. I let the fast-strikers (mostly champions with quicken) run at the enemy, while the mainfore is backing them up with some covering fire (Sluggers, thrown spears, pistols, you name it). At the time, my fist-strikers engage the enemy I pop my feat to cover my mainforce and then put some templates on my firstsrikers, so that the enemys engaging them get -2 on their att-rolls.
But you need to be carfuls, cuz there some enemys oput there that can “see” through clouds (like legion or casters with magesight).

In the end, Jarl is a very fun warlock to play with and he shows you another way to play your trolls.

PS: I already painted Jarl, you can find him here! http://www.beastsofwar.com/rama/minirama-fantasy/jarl-the-devil-of-thornwood/ please vote for him and my other miniatures too!