I got 18 in 1 round on karkand a few days ago trying for the 25. I took a jeep and just spent the entire map driving around looking for stuff to repair, and if I came under attack, I'd try to get away to a safer area, sit in the very back seat of the jeep and repair it, then drive back to the action. I could quite possibly have gotten the 25, if for some reason the guy that was driving our tank for most of the round didnt keep driving off to get wasted when I was trying to repair him. This was on a 64 player server with around 25-30 people playing by the end of the map (round started with 10).
Another way I can think of (which I cant try myself) is to come play on our server (online-assault.com) and play US on Gulf of oman, as I always bomb the crap out of the artillery on that map playing as MEC (no-one else bothers, hence why I cant try it myself), so you'll have a constant stream of stuff to repair, only prob is I'll probly bomb the crap out of you while you repair it :-)