In this challenge, you'll be looking to slaughter half-a-hundred orcs from caragorback. Your two bonus objectives are to kill 6 orcs with Mounted Executions (simple enough), and to wipe out everyone within about two and a half minutes (much trickier, as the time limits always are). In any case, you'll want to begin by dominating the near caragor with Pack Master activated- all the other caragor here will follow you into the wild. Make for nearest group of orcs and tear them apart.
You can get the execution objective easily by remembering to have your mount consume an enemy that's knocked down. Since a single blow knocks them down when you're mounted, you'll have no shortage of opportunities.
There's no penalty for dismounting, but remember you can shoot from caragor-back as well. Use this to dispose of distant, elusive archers, or shielded orcs that are tough to knock down. There's not a lot of easy-to-find elfshot, so make these exceptions rather than the rule.
Move quickly through all three groups to complete the mission.