This is a bit of a nightmare to be honest. If you visit the partner site trueachievements.com there are other tips and guides but here's mine:
Load up a game of walker assault, please note that this trophy is only achievable whilst playing on the imperial team.
Once the rebel forces have activated their uplinks, they will send in the Y-wings to bomb the AT-AT's. This is your ONLY window to gain the trophy.
There are a variety of ways I've read about to gain this one but my method was:
Don't worry about the opposition, kill if required but spend your time hunting around for a aircraft power-up. Once you've collected it you only have a 12 second window to call in the TIE fighter before you loose the ability so call in the air support straight away.
You'll now be flying the TIE fighter and the idea is to stay alive in the skies until the rebels have activated their uplinks. Naturally this is dependent on the rebel players not being complete potato aiming idiots and assuming they CAN activate them.
Once all are active you'll get a message saying that the Y-wings are incoming. Unless you spot them straight away I'd recommend trying to identify and memorize which direction they appear from. I expected there to be LOADS but in fact there were only between 3-4 I think from memory. They will appear on the map, swoop in, drop a bomb... and feck off... they are NOT around for long so your window is short and your accuracy MUST be high.
Try to learn where they spawn from on the map and start from a position in which you'll be chasing them, NOT flying straight towards them. If they fly past you then you'll waste all your time turning and locking on by which time they'll be gone.
Tips:
Lower your air speed by holding
, this will divert power into your weapons making them more powerful and increasing your agility and turning circle allowing for an easier lock on.
Hold
to lock on and try to keep your recital trained on the Y -wing as accurately as possible, once you're locked on let a homing missile go and then STRAIGHT away blast him with your primary weapons on the off chance that the missile doesn't take him out. The missile may not be enough anyway so pump him full of laser!
I would recommend this on a nice open level like Hoth.
People have also used the ION torpedo to take them out but personally I found that very difficult.
You could also try a turret gun (the "Pill-box" type so you're protected, not the dish) but that will rely too much on your accuracy.
Again this is a little luck based as you have to hope that you can find a TIE fighter pickup BEFORE the bombers come in, and also that you don't get shot out of the sky whilst you're waiting for them.
Took me many attempts but it's doable with a little patience.
Good Luck!