Resident Evil 6

ONSLAUGHT-MODE - Urban Chaos

Unless you suck really bad this should come by natural progression.

To increase the time before the combo runs out, shoot the green crystals that float around the maps. Use the red barrels / nades to get up to 15 kills at once.

This video shows you how to get the killer combo achievement in resident evil 6.

Guide for boosting with a friend - Onslaught game mode.

I found this most difficult to understand initially what was going on so hopefully this guide helps explain. First load up up a game of Onslaught with a friend and play any map. Personally I like Mining the Depths for the large open area in the middle to run circles but any will do.

What happens is a player kills enemies and builds up a combo. When that combo ends a number of enemies equal to whatever that combo was get sent to the opponents side e.g player 1 gets a 25 combo, combo ends and 25 enemies are sent to player 2's map. At the same time generic enemies not sent form your boosting partner will also be respawning as you kill them.

The trick to this with a boosting partner is for the player who is going for the achievement to try and keep a decent number of generic enemies alive until your boosting partner builds up a decent combo. Dont go nuts initially, run some circles and importantly locate the green glowing crystal and note its location.

When your partner's combo ends and they send across the additional enemies, kill what you can either with a grenade or by shooting the red barrels when they are close and grouped up (I find Ada's explosive crossbow bolts do NOT work that well for group kills). When you feel the crowd has thinned out by that first explosion shoot the glowing green crystal thing which you noted earlier. Hopefully at this point you should be somewhere between 15-20 kills on your combo. Shooting the green crystal freezes your combo for 30-45 seconds I.e. you can add to it but it won't end if you don't kill anything. Then mop up stragglers to push you from 20 to, hopefully, 30. I find quick shot (press and together) with a shotgun works best for quick kills.

At the same time your partner should have another combo going. Make sure to communicate and tell them to end their combo and send more enemies as soon as that initial crowd is thinned out and you are getting short on stragglers. That should then give you another decent group for a second grenade/red barrel shot. By this point you should be on at least 30 kills. If not pick up more stragglers and shoot the 2nd green crystal on the map (there are always at least 2) to help extend the combo time.