Can be gained on either map, but it will be easier on Ghen Crag. Getting a kill ups the multiplier by 1, and in-between kills there's a short timer. When it runs out, the multiplier lowers by 1. Your combo stays in between waves as well. Since it's possible to get combos up above 90 and 100, this one shouldn't be too bad even if you're not the best at the game. I found the Reclaimer (sniper rifle) to be the best all around weapon, remember it does more damage when scoped. Remember where the red ammo boxes are for each defense point are (they respawn fairly quickly) so you're always topped up on ammo.