You need to complete all the waves from 1-30 all the way through without losing all your lives. (You need each of the boxes up the top to light up)
Firstly you want to setup a game of Infestation Mode. Now set the difficulty to Casual and the level to Black Market as it is a close quarter map. So those aliens won't be able to shoot you from a very far range.
Now choose the lowest level Starting Wave you haven't completed.
I would recommend having the Shell upgrade. Everytime you get on low health simply just drop the shell down and melee everything that comes inside until your health is back up.
Your only danger in Infestation mode will be the Berserkers as they love to Suicide bomb you when they get low health. Best technique to kill them is to just use the shotgun and aim for the head as they run at you and you can possibly blow him up and all the aliens around him.
You will also come across the Monolith from time to time which buffs all the Aliens surrounding it to double health. Can be quite a pain for the Berserkers as they already have enough health so dispatch of the Monolith quickly
Waves 1-5, 11-15 and 21-25 are very fast and have bearly any aliens in them at all (Around 2 minutes each)
Only hard waves are 10, 20 and 30 as they are around 10 minutes long and have waves that are purely just 10 Berserkers.
Keep in mind that you have around 10+ lives so you can die, just not too much.
If you do manage to die too much and the level ends just simply go back to the menu and choose the level you failed and launch it again.
Little Extra Note: You don't have to do this in coop as i done by myself Waves 1-30 and only died 3 times. If you bring a friend it can bypass the time and make it 2x more fun.
Have fun and if you have any troubles post a message in the comment section below.