These achievements will be a grind. You gain level by gaining points. You get points for playing online and the campaign. Points are based on how well you do, mainly how much you kill, but also other things like getting good job awards. Also, it appears that if you're playing in campaign and hosting a game, for some odd reason you will not receive as many points as the people playing with you.
There are two chapters that are good for grinding out some career points and/or gaining credits from box drops. If you still need credits for the slot machine then play 5-3 for sure, as you get way more box drops than any part in the game. In my opinion, once you have the slot machine maxed out, I think playing 5-2 for points is easier. On 5-2 there are two sections, but you can put it on Extreme and easily pass it every time with two people. Once you get it down, 5-2 can be done in under ten minutes. On 5-3 Extreme sometimes it gets hairy trying to board the other ship and get the timer, its much harder to do this every time without dieing, especially with only one or two people.
As long as the NON HOST player kills the boss or ship (depending on which chapter you select) and the difficulty is on Extreme both of theses chapters will net close to 70,000 points.
If you're playing for credits than difficulty does NOT matter. When you're trying to gain career points to level up your characters difficulty DOES matter. The higher the difficulty the more points you will earn. Boosting for career points with more than two people is a waste of time, seeing how only one person will be getting most of the points each time. Boost with as many people as possible if you're playing 5-3 for the boxes. Having AI on doesn't make a difference as far as points earned. I suggest keeping them on as they do very little damage to anything that matters and act as good distractions for the enemies.
Before you start
-Host equips T-ENG Supply ability (generates T-ENG quicker)
-Player 2 equips Superstar ability (double points earned)
-The person who is NOT hosting gets on the laser cannon
-The host should shoot the laser cannon anywhere with T-ENG while its in fire mode, keep doing this until the cannon charges to level 2 (listen closely for 2 unique sounds, indicating the cannon has charged a level).
-The host keeps charging the cannon so every shot Player 2 takes is fully charged
-Player 2 should take out the cannon and four wings of the ship. It's also a good idea to destroy the Offsprey that will try to fly onto your ship, as well as the turrets on the enemies ship (at least the ones on the side you will board it from).
-With the enemy ship's defenses down, you need to board it and activate a datapost. While you're doing this they will send enemies to board your ship and activate a post. Whoever gets their timer down from 100 first wins. If the enemy gets their timer down first you will fail the mission losing all points earned.
-I think its easier to have one guy stay on defense and send the other guy over to clear out the enemy ship and activate the post. Once the enemy post has been activated the guy playing defense should go over and help defend the enemy post. As long as you start the timer first and don't let the enemies stop it, you will win.
-Once the timer final runs out, stay in that room and collect boxes if you need them, they will continually respawn. If you have enough lives to spare both guys can stay in that room and collect boxes until the timer reaches zero and the enemy ship blows up. If you don't have enough lives you will have to leave the enemy ship with about 15-20 seconds left on the timer, enough time to jump back on your ship before the enemy ship goes BOOM. If you die and run out of lives at this point, all boxes and points earned will be lost and you will have to start over.
Before you start
-Host should equip VS Takedown ability and T-ENG supply
-Player 2 should equip Superstar ability
-Defend the ship from enemies coming to activate switches on the hangar door that is located down below.
-Have the host, with VS takedown jump on the turret located in the front of the ship and shoot down all the incoming Offspreys. Player 2 can also help. Both of you together should be able to kill most of the choppers before they get close or drop off guys.
-There will be a point when four enemy soldiers jump onto your ship from the front. At this point jump down near the hangar door, by the switches you are defending. You will have one good push from the enemies here. Do not let them get close to the switches. On Extreme they activate them quickly! Once the push is over jump back up and take out the last couple Offsprey.
-Player 2 goes straight for the laser cannon while the host stands in T-ENG generators and builds up T-ENG. Charge the laser cannon up to level 2, same way as in Chapter 5-3.
*note - while the hosts waits for the boss to come he can grab the Gatling gun and kill about 50 Akrid flying around, a nice little boost towards your Akrid kills.
-Player 2 should take out all four legs first with fully charged laser shots. If you aim for the back legs, sometimes you can get two legs at once. With the legs done and usually one more shot on the boss, he will attach itself to the ship. Make sure your laser is fully charged and blast him right in the eyes. This shot will kill him taking about half of his health away.