If he uses vehicles it is best to go with Meltaguns if the Squads are going to be going close or Lascannons/Missile Launchers if you want to shoot them out before they get too close to you.
Well this isn't strictly true.
Eldar vehicles are all fast skimmers, which means that you can generally only ever get glancing hits on them.
But they also never have an AV greater than 12.
So your best bet against the vehicles is massed mid-strength firepower. Plasma guns, autocannon, assaultcannon, heavy bolters.
You really want the greater number of shots that these weapons will give you, whilst the lower S will still easily glance his low armour.
Even bolters will ruin the day of vypers and warwalkers since they are both AV10 open topped.
The high S and low AP of Lascannon, missiles and melta are all a total waste...while the low rate of fire will hurt you more than him in the end.
Dizzy