I thought I'd never get these but I found an easy way for both. Wait for King Of The Hill on The Pit (it should come up often). Get the sword and go below where it spawns, hiding on the middle ramp. When the hill moves to your location, you'll be outside but right next to it. When the hill chasers show up, slice them. This is actually how I got my Overkill. You can also get a super easy Overkill on the same map, playing the same game, and the same hill location. This time get on a turret and mow the hill chasers down. You may end up with a Killing Frenzy as well.
I should mention that I suck at Halo and these ended up very easy with this method.