You have to go by where the yellow Invisibility pick-up is.
Just make sure to strafe left and right most
of the run to see when
people come from all the doorways.
Just try your best to get that many each minute;
Basically, running around frantically can work, however,
I would just camp for an easy score over 25.
The place to camp would be right in the action (most of the time).
If you get it, the girls will not pay attention to you,
sometimes they won't even come near you.
However,
if you stand near it, most of the time at the start,
they will try to grab it, and this will be your chance to kill them.
It's just a matter of surviving, as there are a lot of girls in
the level, and many times they will be very small,
and extremely hard to hit.
It's okay to die, but try not to;
if you do, just try your best to get back to where
the yellow pick-up is/was.
You have 3 minutes;
since you need 25 kills,
you should only need 9 kills a minute to score 27,
which is over the platinum requirement.
although I can say,
I have personally gotten 14 kills in 30 seconds, so as many as 28
kills a minute is possible
(but believe me, if you are a beginner,
don't count on 14 kills in 30 seconds happening too often, if ever.)