If you're just starting this now I say you still have time.
You can track your progress here:https://gearsofwar.com/en-us/my-rewards
Don't forget to redeem for your skin once you get this done.
If you are or aren't familiar with the last Challenge they did like this, that was for getting 6,000 kills. This time they changed it up though as now downs also count.
In theory you would just have to down 4,500 enemies and then get the kill on them. That's still a lot of work and is very unlikely to happen so evenly. It's possible other people will kill the enemies while you're trying to down them or they'll get the kill after you get the down.
Try not to worry so much what other players are doing and just focus on getting your own progress.
I think all the ranked playlists (both core and competitive) should count as does co-op versus AI.
I don't think quick match or competitive warmup count.
Not sure about the Gnasher only KOTH special event going on.
Ideally you'll just be playing casual/normal AI matches anyway.
Most of you probably already know but for those that don't, the best way to grind kills is in KOTH (King of the Hill) mode. When you select the versus co-op playlist though generally you don't get a say in what mode it loads you in to so you have a choice, either play out the match and hope that the next mode/map vote has a KOTH option or to quit out and then select a different co-op difficulty. Honestly it's not a major difference going from casual to normal but I wouldn't go any higher than that. Enemies will take more and deal more damage. You also get less human players to play with which I'll explain why is an issue.
The reason KOTH is so good is because you get infinite spawns for yourself and for your kill fodder enemies, then you can also draw out the match for longer periods of time compared to other modes. Length of the match is important because that's less down time for you not getting kills by having to look at post game scoreboards, voting and loading screens.
If you have bots on your team, they will always go for the hill and capture it which will give you time to the target score. You want to avoid hitting the target. If you have five humans players it's easy to simply not capture the hill and to keep the enemy off but you have no way to keep friendly AI away from it.
You shouldn't have to go out of your way to make a premade team as people will join in and out constantly on the lower difficulties, most of them are going for this Challenge too.
Just something to be aware of.
Now for actually grinding the kills once you've ended up in the desired mode will all be up to preference.
One fairly common tactic you'll see is people running straight up to enemies with the Enforcer in their spawn loadout and then emptying a clip into them to then finish them off however which way. While that is one good idea and you would be getting a +2 towards your progress, the time it takes to get the down and then the kill is enough time for someone with a Gnasher to also run up and kill the enemy before you can get the down and/or kill.
Ideally the best way to go is Gnasher. You can run up and clear a whole team of enemies in the time it would take an Enforcer to get the one down and kill.
You'll also want to spam those power weapons. I notice a lot of people don't go for things like the Longshot sniper rifle or the Torque Bow. They're actually very good as you can kill a bot coming out of spawn the second their spawn protection wears off without actually having to waste time getting much closer and the sooner they die means the sooner they respawn for you to kill them again.
The second your power weapon empties, make sure to grab an enemy Lancer off the ground so the power weapon will respawn faster.
Since I mention the Lancer, it can be kind of a good weapon to attack enemies if you run out of Gnasher or while you're waiting for a power weapon to come back.
I would start off using the Gnasher and fully use it before pulling out the Lancer though unless you're a more passive/support player, then maybe sitting back from a central elevated position would be your best bet but you'll get progress a lot less slower.
Lastly I'm gonna throw out some stats.
You can check percentage progress in the achievement areas on the Xbox, every 1% is about 90 kills.
If you're being efficient (and playing with lesser players on your team) you should be able to get about 10% in roughly three hours. I'm not the best at math but that's about any combination of 5 kills and/or downs a minute. Getting this Challenge done should be under thirty hours of playing, while you still have time I would try to setup your schedule into smaller play sessions as much as possible to break up the monotony.I would say "have fun", but nah, you won't have.