For gateway I used the plus 2 Liandra and told everyone to hold position on the minigun in the waterfall section and ended up winning by like 15.
Deimos I had everyone hold position in the link gun room and put them in a triangle formation so they wouldn't all die to 1 rocket and I stayed behind the glass that has the 2 health spots. I was up by like 20 and left my system to make a pizza and won by like 10.
The last level against Ashka I stayed around the shield belt and used the link gun to shoot across the way and won 20-17 on my third try.
That's how I beat it by myself I could have got lucky on some stuff though.