Tactical Squad Combat Squad and put the lascannon so that you'll get at least 1 lascannon in the battlewagons side a turn.
Turn 1 : Thunderfire shoots the lootas. Lascannons (and possibly autocannons) shoot the Battlewagon. Melta, sergeant combat squad move straight towards the battlewagon ending 14 inches away.
Orks : Battlewagon moves straight towards the Pred (and thus straight towards the Melta). Lootas will probably either shoot the Melta Squad or the pred. If they take the bait and the orks disembark to charge the Melta Squad then the lootas won't be shooting their target. If for some reason the orks dissembark. We are probably the Pred incapacitated but no other damage.
Turn 2 : Melta Squad Move so they are at the side (if they survived and shoot the side). If the pred can fire it does so afterwards (if the BW is destroyed then shoot the lootas). Lascannon squad shoot the BW (if it still survives), if not then they Boltgun/Lascaanon the Orks. If the Orks are out of the BW then the Thunderfire Cannon shoots the disembarked orks. If not then it shoots the Lootas.
Orks : Any remaining BW Orks will be able to WAAAGH practically everything you have so prepare to lose a unit. Lootas will incapacitate the Pred most likely.
Turn 3 : Remaining tactical squad can finish of the BW orks. TF/Pred Should finish of Lootas.
Out of intereast where are the second troops choices and
HQ's as if they are already dead 1 Turns already been played so if you don't kill the BW in your headstart you lose as they waagh and hit your lines straight away.