HARDCORE MODE.
To get acces to harcore mode you need to complete mission 10 in story mode.
Then you must choose one of the modes to abtain this trophy.
You can't re-do missions to farm money or ranking, so make sure you get all the salvage collectibles for parts. No parts nor money carry over from normal mode.
You got a limited lives in every mode.
In my opinion the easiest ones are:
F: Gambler
E: Mr Confident
J: No Fear
F: Gambler
AC: 5
In this mode you deal 10x more damage, but enemies deal 5x more to you, so in my opinion is a fair trade, best way is to use long range weapons, such as laser rifles or sniper rifles and kill enemies in one shot from afar. Also CWS is a lifesaver from many situations ;)
E: Mr Confident
AC: 1
In this mode everything is the same as in normal game, except enemies deal 0,7x damage to you, so its even easier than a normal campaign. But since you got only one life it's good to check harder mission first in normal story to be preapered for it!
J: No Fear
AC: 10
This one is tricky, effective shots deal 5x damage to you, and 1,5x damage to your enemies, but ineffective shots deal only 0,2x damage, so if you have a good armor they won't do almost any damage. The best way is to use heavy bipedal since he got decent defense on almost everything, but you can say "there are weapons that break through any armor, like sniper rifles for example", No worries you can take a shield with you that can protect you from everything, except charge attacks.
Another debufs here are; 0,3x reward from mission, in exchange for 5x fater fill of Rank points, so you will get best stuff in shop very early, you got to use your pocket wisely!