I think you are seriously underestimating mob spawn time.

You have to do this out in the open. You can kill your group of mobs, get the exp on 5 chars, great. Then wait. And wait. And wait. Then do it again for 5 more. And wait. And wait.
The spawns in that guide are tiny. Like 10 mobs usually, and they have to be specific melee types to aoe down efficiently..
Level them in groups of 5, then play the "endgame" you are aiming for. It is not going to work any other way, but then again, you are not going to listen and try it yourself regardless.

So best of luck I guess.