Add defeated AC emblems
Press Select + Start when the victory message appears in the arena to add the defeated AC's emblem to your list

Bonus parts
For every mission you get an "A" or "S" rank on, you will get a special bonus part. The following are the other requirements to get bonus parts at the store.

MRL-MM/001: Reach AC rank "D"
MWGG-HNRL-100: Reach AC rank "C"
KAW-SAMURAI2: Reach AC rank "B"
MAM-MX/MDD: Reach AC rank "A"
MBT-NI/GULL: Reach AC rank "S"
CWI-DD-30: Defeat E-21 at the Arena
RGI-KDA01: Defeat E-15 at the Arena
KWG-NHZL30 : Defeat E-8 at the Arena
CWR-M70: Defeat D-7 at the Arena
CWM-BM60-1 : Defeat C-7 at the Arena
MWG-SRFE/8: Defeat B-4 at the Arena
MCL-SS/RAY: Defeat A-3 at the Arena
MWG-MG/FINGER : Defeat A-1 at the Arena
CWM-HA40-8: Defeat rank 1 in Extra Arena
MWG-HGB/108: Rank "S" in all missions
KGP-ZXV1: Complete all missions
CCL-02-EI: Complete 10 missions from Crest
MCM-MI/008: Complete 13 missions from Mirage
KEEP-ELIX: Complete 7 missions from Kisaragi
Op-Intensify (All Abilities): Obtain 99% completion of the game

Defeating 3 ACs.
Okay, there's (in my opinion) a very tough mission, in which you need to fight 3 ranking ACs without a break. The first AC, you fight alone, so he's not that tough, but its on an elevator, and you start out on the bottom, while he starts on top. Get the height advantage and he'll be a pushover.

The next two are very tough, because you need to fight them together. I had a little plan of my own, tough.

Buy a Chain gun for your back weapon (for either side or both), and get into the nearest corner. The corner may not seem like such a nice place, but since one AC comes in the fight slightly later, and from the back, it can prove VERY effective. It proves effective, because the strategy for most computer ACs is to get behind you and attack.

In the corner, you can see AND attack everything

Defeating the super ACs.
Okay, there will be a few mission where you wll encounter some giant-sized AC, with an enormous gun. These are pretty hard to defeat, but some tips can make him much easier.

1- When he shoots at you with his semi-auto rocket launcher, stafle in any clear direction. If you have any speed at all, you should barely outrun it, without getting hurt. Shoot him while you're stafling to inflict major damage.

2- Despite his size and strength, these super ACs are very fast, so Beam Sabers aren't very effective unless they come to you.

3- Don't let his size intimidate you. He may be big, but he's easy

Defend Lawdas Factory mission: Hidden part
When the big laser destroys the missile in the center of the map, jump down and look for the part. The part is an extension CWEM-RT16

Defend Mirage HQ mission: Hidden part
When the mission starts, turn around and boost towards the helicopter landing site. Look around the landing site to find the back stealth missile, MWM-SM36


Defend Solar Power Plant mission: Hidden Part
After you defeat all the enemies on the outside of the power plant, you should get a notice about enemies inside the plant after you enter. Clear the first floor. When you go down the ramp at the end of the first part, you will see a fork. Go to the left. There should be the part at the end of the room. The room is on fire. The hidden part is the KWEL-EJ22 anti-missile