How do I play her top? Bait a duel and pull out a giant mantra'd shield+heal? What should I be building?
Well her early game burst is alot so try to mantra shield them down (super easy because if they're melee they need to last hit your minions) then do a shield flash ignite q combo with w thrown in just in case you need to auto attack.
As for build, go mostly reactionary, but a max tanky AP karma usually works well because her shield can soak up so much damage. I usually go
I will not change this sig until Alterac Valley is reverted pre-BC http://us.battle.net/wow/en/forum/topic/969937114
Step 1) You wait until the rework.
Step 2) You fight with the other two people on your team who want to play her.
Step 3) You dodge.
Step 4) You try again.
Step 5) You dodge.
Step 6) You play Karma (post-rework).
I saw the storm reach from the sky to the ground
As dark as the day was bright
From: Theshamen | #002
I would go mid but I think she gets outclassed there. Karma top probably works better because of the brusiers.
To play her mid, you just pretend you're morde. Shove lane forever and build up some moneys
And then John was a zombie.
I like to get early Chalice followed by Morellonomicon and Grail. CDR and mana regen are extremely important on her and with 40% you might be surprised by how useful she can be in a teamfight.
Offtank Karma is pretty awesome. Merc Treads, RoA, Abyssal, Frozen Heart, Liandry's, Grail, in whatever order makes sense. The trick with Karma is that she can build a lot tankier than most mages because she can make a much better use of her passive when she can take a hit or two; it's a lot more fun to run around at 30% health when you've got a butt-ton of armor/MR and 3000 max HP to begin with. If you're not meaty enough, then running around at low health is just gonna allow any carry to one-shot you by left-clicking on you to check your stats window.
Heck, if that's a bit too beefy for your style, think of building her like you would Swain, or Cho'gath, or Rumble (but with mana). She's really up there in terms of potential durability. Besides, if you're running top lane, your team is gonna need you to be able to take a bit of damage, especially if you've got a jungler who is not very tanky, so a glass cannon build would probably be quite ruinous.
"ˇViva La Revolución!"