This achievement is fairly easy, but I was messed up several times by the little quirky details. Here are some specific things I learned while going for this achievement:
I did NOT get the achievement in Large Team Death Match. I was on the loosing side, but had a battle rating of 106 and no achievement. I had to get it on an FFA match. Here are a few things to help you boost your battle rating:
Knife and Bow kills count for a lot: If you only get a couple of kills with your knife and bow, unless you had terrible accuracy, you are almost guaranteed the 100 battle rating. Using weapons that take more skill to kill with is probably the most important factor.
Accuracy really counts too: I've done all of the above, bit with an 11% accuracy rate I would only get a battle rating of 90 or so, and that was with more kills than deaths. Accuracy is probably the 2nd most important factor here.
Kill/death ratio hardly matters: I got the aforementioned 106 rating having 3 kills and 9 deaths.
Using different weapons seems to help: I have noticed that even with a decent K/D my battle rating is lower when I stick to one weapon in a game.
Suicides count against you: Just...don't.
Not sure about dino kills, either killing them or getting killed by them. Also swinging your knife and missing doesn't seem to count against you in accuracy rating.
Hope this is helpful!