To level defense, go to the "silithid swarmers" south of Camp T in the barrens. Kill the one "swarm" bug but leave the scorpid alive. Wait for 50+ tiny bugs to spawn, then kill the scorpid. They will all hit for 1. If you have any sort of heal interruption protection you can heal yourself. If you don't mind potting, you can usually just pot every two minutes to keep your health topped off. If you've got Uther's Strength (BOE trinket) you'll take no damage at all. One AOE, thorns, ret aura, pet on defensive, etc. to kill them off if you just want to eat up and pull again.

You can go from 1-"max-5" defense in ~3 minutes. To get the last few points it usually takes 5-10 minutes.