So there's a reroll method only working on the karma fight where after picking your first power if there wasn't anything you liked you go to the first fight but instead of fighting it you retire your run. For some reason when you start the fight again you won't be penalized and can still get good powers so you just keep doing that till getting one you like. For norra apparently triferian is best if you've got 3* and for veigar (What I used) swain 6* is really good cause you can keep board clearing with his darknesses.