1. Accept the Protect the Guardian quest (NEVER TURN IT IN)
2. Start a new game, re-roll the world and waypoint to The Temple Steppes.
3. On trash mobs charge your bar until it procs.
4. Don't attack anymore mobs unless you have to, the speed buff will stay on you.
5. Because you have the Guardian quest you can see the waypoint on the undiscovered map, allowing you to find his place easily.
6. Run through both levels of his lair
7. Outside of his door tell your friend to teleport to you.
8. Kill Grell, loot his body/chests and then leave the game afterwards. Make sure you don't complete the quest.