dealing with multiple priority threats

my last couple of games I've played against my brother at thousand points (48"*48" table)
he fielded his Necron destroyer cult which looks something like this:
1 Destroyer lord, 2X 5 Destroyers, 20 Warriors and 3 wraith

Although this doesn't look like an über competitive list he gave me a pretty hard time and he destroyed my army. His basic battle plan went something like this: send the wraith down the middle of the table with a unit of destroyers on both flanks while the warriors stay behind them and capture objectives.
my reaction to this would usually be to keep distance and focus on one of the units until it's dead. (which usually doesn't happen)

since the table is 48"*48" I sometimes can't even prevent a charge second turn. he then starts killing some of my transports and grinds my army in a few turns until there is almost nothing left.

I haven't tried to make a list specially designed to kill his although the time I took five gunboats I killed his army but the five gunboat army was really boring to play and woulden't perform to well against a vehicle heavy list.

thanks in advance for any advice

P.S. we play the maelstrom of war missions.

Deploy everything in one corner and shoot anything that gets close, concentrating all your fire into a single unit until it is dead. Turn two, you turbo-boost/flat-out to the opposite corner. Rinse and repeat.

You might want to feed the wraiths a cheap unit to tie them up in assault for a turn and give you some breathing space. Wyches might actually be useful!

actually I hadn't thought of that yet. (I usually play Chaos who face their threats head on and stamp on it until it's dead)

I'll try this out in our next battle and try to report back what happened.

but whilst playing a maelstrom of war mission I think you are going to lose a lot of points to midfield objectives but by careful objective placement(in the corners ) I think you can handle with this.

If you place your objectives with one in the corner you deploy in and the other two as far over the other side of your deployment zone as possible you will force your opponent to go after your units or go after the objectives or split his forces.

Ignore the warriors, shoot the wraiths first then focus on the destroyers. Poison is powerful against him.