Whenever you play a character like this, you have to weigh the damage output of a spell against the utility of the spell. I.e. should you get cdr or ap? With karma, there is no chance the utility is worth it. Moreover, due to her mantra taking 25 seconds and only stacking twice, there is really no point in cdr. You want ap and magic pen asap, and you want your shield maxed before anything else. I don't even bother with the slow until late game for ganks. With this build you can easily take mid and even zone fairly well.

Never use mantra to heal unless in a dire situation. You should be using it as much as possible to keep your enemies weak. It's actually better for karma not to be at full life due to her passive, so use the shield to maintain approximately half health, no more no less. Mantra shield minions every time your enemy gets close to minions (it has a surprisingly large range). Once they are low, and if you are low too (this makes you a tempting target but also effectively doubles your damage), run up, mantra shield and fan and they should die.

Later game, you might seem like a ks, but mantra shield on your tanks and you will rack up the meijas. If they complain, who cares, you can carry the team.