Manta To defend your self against manta attacks, try to get some avril rockets wait till they try to splat you coming right at you so you can kill them from a few meters. Works 9 out of 10 without getting killed yourself.
If you dont have a an Avril launcher use an shock rifle and use the alt fire to create a web of explosive balls around you. works 5 out of 10 for me.
Viperian
OK, general vehicles defence tips: Firstly, know how to use the avril well. Check the avril page for tips. Essentially, fire a rocket in the air first, then lock on to a vehicle so the missile has less distance to travel and is harder to dodge. The avril is your main vehicle defence.
Secondly, don't underestimate the ordinary rocket launcher in anti-vehicle situations! It doesn't pack the same punch as the avril, nor can it lock on as well, but when an avril is not available it can be very useful as a full salvo of rockets can really throw the vehicle around - especially if you fire them in a spiral so that they all hit from different directions.
The other two main anti-vehicle weapons are the shock rifle, as described above, by spamming shock combos. This is most useful against mantas and scorpions as they are lightly armoured and get thrown about easily. When using against mantas, detonate the shock ball directly below the manta for maximum effect. The flak cannon can be used against just about anything. Ducking and blasting up at an incoming manta can do damage very quickly, as can firing at a scorpion or hellbender as they drive past. The flak is also useful against heavy targets like leviathans and goliaths. Run around the vehicle, firing an avril (which takes an age to reload) then switching to flak cannon for 3 shots or so before switching back to the avril (which is now just ready to fire) and firing another shot.
Finally, know how to use vehicles against each other. Mantas and scorpions can easily strafe around a leviathan or goliath and blast away, and can do the same, to a lesser extent, to hellbenders. Scorpions are the ultimate defence against mantas, whose air intakes suck in the scorpions primary fire. Raptors can butcher goliaths and leviathans by flying directly above them where they cannot fire back, and turrets and hellbenders' rear turrets are great at taking out light vehicles (mantas, scorpions, raptors and other hellbenders) from afar.