All Dressed Up and Lifer Achievement Solution
Exp needed for Rank 8: 793,500
For all four armors, exp needed is 3,174,000.
The maximum score/exp you can gain from playing a level is 65,500. I've confirmed this on a few levels on both Hard and Inferno difficulties. Predalien90 in the comments confirms Normal has the same cap. 65K is attainable with patience along with any spawn nests, hills, or drop ships.
If grinding to max score from the start of a class, you would need to play ~12.1 levels, or 48 (plus a bit) total sessions for all armors.
Score/exp on Normal, Hard and Inferno for regular spawning enemies. (If score/exp on Normal is X, Hard is X*3, and Inferno is X*9.)
Ticks: 1, 3, 9
Ants: 5, 15, 45
Spiders: 6, 18, 54
Gunships: 6, 18, 54
Metal Ants: 7, 21, 63
Metal Spiders: 8, 24, 72
Some good grinding spots.
The Cube: Level 2-5. Towards the end, before you fight the four elite "bosses", there is a crater with two broken silos. Here, 3 Metal Ant hills spew out ~15 ants per go. If bored of ants, after you seal off these three, a Metal Spider nest appears, and spawns 16 spiders.
Invasion Alert: Level 1-1. There are two separate ant hills. The first spawns 6 at a time, the second spawns 12.
Via Predalien90 in the comments, and Soulrift X on XBL...
Alternately, if you "speed run" Level 1-1 on Inferno with a higher ranking character and good weapons, you can earn 20K-25K in a short amount of time.