After finishing the campain on hard and collecting all audio logs, you'll probably have only have to grind kills and level the other chassis (classes). If you only have one of those ramaining, please follow the specific guides
I found that Mission 2-1 is the most effective level to combine both.
You will get 150-200 kills and 15,500 XP in about 2.5 minutes by following the steps below:
1) Start Mission 2-1 on easy
2) Head to the mission pointer (presss
to run) and kill some smaller enemies while passing them.
3) On your way you will meet Garris (huge guy) for the first time. Don't kill him, but shoot him for 1-2 seconds (See notes at the end!)
4) Head further north-north-east and enter the barn you will see (glowing entry).
5) Press the button in the back of the barn. This will trigger 130 mini spiders, which will just drop in front of you and can be destroyed within seconds.
6) After the spiders stopped dropping, leave the barn and continue to the mission marker. Don't stop for enemies, but kill the bugs on your way
7) Collect the mission objective and leave the level through the exit further east. Notes:
- When heading straight to the barn without killing some smaller enemies and/or damaging Garris, the spiders won't spawn or very slowly
-Skip all conversations by pressing X!!! Otherwise you can't collect the objective and leave the level until the conversations are over