Completion Strategy:

Gulp. Another one of those 'it's hard to give a strategy' levels.
Almost everything in this level is random, so I probably won't be able to help you much, but I'll do my best.
You, of course, want to use the SBP-90 X2.
It is supperior to all other weapons (if you can't get your hands on this, Tactical 12-Gauge, and if you can't get THAT, well, you'll have to try using the Plasma Autorifle's explosives).
It The SBP-90 X2 can be found in two places (but what else is new :/).
In the middle of the HUGE arena thing, where the Challenge 'Sergio's Last Stand' is located. You know what I mean ;).
The second place is in the middle of the carosel in one of the tents.
It's not too hard to find.
Try and get one of those, but it really doesn't matter, so long as your getting kills, and fast.

- Neo

Platinum/Fast Time Strategy:

Errr, I have no idea how I'm going to explain how to get platinum on this.
Luckily for me, 55 kills isn't that hard to get, as I have 81 :D!,br> Ok, well, as previously mentioned, you want to get the SBP-90 X2, it can take down any guy almost instantly.
The Tactical 12-Gauge will do, but you'll have to be at close range for the spray of bullets to hit.
You usually want to stay out of the huge arena if your going for kills, sure, you can get lucky and have HEAPS of guys in there, I've seen it happen.
But most of the time nobody will be there because it's a lot further away from the rest of the level :(.
A good place to get kills is mainly in the small tent in the middle of the grounds, and outside in the open.
But since it's totally luck, again, I can't be any more exact then that.

Good spawn points? Another topic that is hard to answer.
I can't really give you any, because even when you get the SAME spawn point, it can still be random! ARG!
Mess around a bit by yourself to figure out where your best starting points are.
You should have platinum in no time, if not already :p.

- Neo