At the 300 mark you'll need to be going into master and start playing around with mods that suit getting a-grade rewards. Firstly, make sure you equip new gear and the highest level you can... this will increase the score multiplier and you'll have a better chance for a-grade without doing much else. Stepping up a level will generally mean you won't need to put on too many negative mods, if any at all.. in fact it's already a-grade, see if you can get away with putting positive mods on. After level increases, you'll just have to start putting negative mods on.