Simply beat the game on hard.
This game is not too bad on a hard playthrough. Takes about 8hrs or so. The most difficult parts are probably going to be the Target bases, only because their guys have great guns/armour.
Some general tips:
1) I found the best weapon combo to be the G36/Tavor or the Kaluga/G36. On hard, the enemies do quite a bit of damage, so you want to stay away from them as much as possible. The Tavor is good for headshots, and the G36 is an excellent all-rounder. Once you get the Kaluga, don't look back (except when they take it from you for a couple stages). It shreds pretty much everything. The pistols/shotgun are worthless. If you prefer the Minimi, take it instead, but I found accuracy to be the deciding factor. The energy shotgun/rocket launcher can be useful in large arenas, but the slow reload speed and poor collision detection (thinking you did not lean far enough out of cover) ruined it for me.
2) There are very few infispawns, and they are only 1/2 guys at a time. Very manageable.
3) If you can spare them, explosive bullets shred heavies, as does the Kaluga. It is not as effective against Target elites through, so impact grenades work well, as does the energy shotgun/launcher (if you are using it, I just shot them a lot with the Kaluga)
4) In large arenas, a good tactic is simply to kite the opposition. The AI is very poor at following you quickly, so killing a couple from range, and sprinting to the other side to repeat works very well. This is eespecially useful in the datacore arena near the end.
5) Learn mine placement. After dying in a few arenas, you know the spawn pattern and can place mines accordingly, killing a lot of enemies quickly.
6) Be careful with the doors in Target bases - on two occasions I had to restart the stage because I moved through an open door, retreated when I got shot, had the door close and lock in front of me (as it was designed to keep you in the room).
7) EDIT: As mentioned in the comments, the beat the last boss, you need to damage it (with guns/mines/grenades), upon which it drops intel. After getting enough intel for the pulse wave, hit the boss with it, then dodge airstrikes on your way to activate the SAM site at the rear of the arena (via the computer panel on the back of it). This nets you a checkpoint as well. Repeat twice more to win. I found the Kaluga combined with mines and an impact grenade or two the best way to damage the boss, as it will shred you up close with SMG's if you let it.
8) Keep at it. As I said above - spawn positions are static and the enemies fairly dumb. Good luck!
If anyone has any other tips, please put them in the comments and I'll add them. Thanks!