Make sure you have saved data as you enter Anvil's Ford area, that way if you don't get this achievement you can reload and attempt again.
This fighting happened directly after you receive the One Tough Cookie achievement
There will be a counter upper middle right to tell you who is winning
War on left, Ulthane on right.
Try to finish them off with the B, otherwise Ulthane likes to pick them up and finish them off, thus stealing your kills.
Proceed through until you get to the single Angel carrying the huge blaster, once he is dead, walk over to the blaster and B(action) it.
First take out any Angels that are close, then pick off all the Angels fighting Ulthane, this will help you keep your score up ahead of his.
Continue proceeding through with this weapon (LT, powered up explosion, RT laser shots)
The blaster(i believe it was called redemption) is your best friend up until you have to jump across and meet up with Ulthane(remember before you jump across, use the growth x2 for soul chest and wrath stone chest)
Once you get to the next door that Ulthane should hold open, you will get this achievement.
Ancient Tides has said that if you tie at 40-40 you can still pop the achievement.