Botani is probably the easiest to farm. Just have to kill Acanthus 5 times over, he gives you 20% each time. If you have the bodyguard that ports you back to your garrison, that will save you from using your hearth. Also, if you've done the garrison campaign, you'll have the mole machine out front to port to Gorgrond. If not, just take the portal to Tanaan and fly from there. All up, it takes me about 10-15 minutes to farm each invasion.