The only new vehicle in multiplayer is the Mantis... and it is a hell of a lot of fun. I have jumped into the Mantis probably a dozen times so far... here's what I'm thinking for a basic strategy.
Fighting With A Mantis
When using the Mantis you should primarily be using the turret and saving the rockets for closer calls. Use the turret on ground pounders from a moderate distance and they'll drop pretty quick. If a ground pounder gets close enough for you to worry about getting stunned use the rockets with the turret to cut them down quicker. Also, I like having the rockets fully loaded so I can fire multiple rockets if necessary. When you fire all 5 it will automatically reload, (You cannot reload without firing them all), - if you can help it don't leave just 1 or 2 rockets ready to fire.
Pace yourself when firing with the turret it does overheat quickly, quicker than the warthog turret. If your turret overheats and you're getting overwhelmed use the rockets that you've saved to backup your overheated turret.
Using the Mantis to fight against other vehicles is essential. I like overwhelming the enemy vehicle with both the turret and rockets when the vehicle is within range. If you can unload all of the rockets into an enemy Warthog along with constant turret fire you should be able to shred the hog.
Long range combat with the Mantis is really only reliable as team shooting. Don't even bother firing the rockets long range unless you can lead your shot with skill.
I have yet to use the Mantis (Smash!?) stomp with any success. I attempted to stomp a ground pounder who was getting close and I ended up getting boarded still. You cannot stomp to one side or the other, it will always be a straight forward stomp. I don't think attempting to stomp someone directly below will work. When using the Mantis Smash... It takes 2 stomps to kill a Warthog. 1 stomp to kill a Ghost or Mongoose. Destroying vehicles with a stomp WILL damage your Mantis! I think it's so extreme that if you have some damage on you before you stomp a hog you will blow up with it.
The Mantis is MOST VULNERABLE to the plasma pistol in my opinion. When the Mantis is stunned you cannot fire like you can with the hog. You are completely frozen and helpless until it wears off. Team support would be very effective in maintaining your Mantis. Once you are boarded consider yourself done - I have attempted shooting nearby walls to kill the boarder like I've done in the past with a Wraith or Tank and it doesn't work since you can only fire forwards and the boarder will always be on the back. I've yet to see a teammate shoot a boarder off of the Mantis and have the Mantis still survive - I'm sure it's possible.
I tested out the Mantis against a few undamaged vehicles... below is what it took for a total kill. Using the turret or team shooting will speed up the process.
Warthog - 4 Rockets
Banshee - 3 Rockets
Mongoose - 4 Rockets
Ghost - 3 Rockets
Fighting Against A Mantis
Team shooting is very effective against a Mantis, (I know this because I've been shredded pretty quickly in a Mantis against only ground pounders.) If you can involve a vehicle in fighting a Mantis that will increase your chances of success. Involving more than 1 vehicle with a coordinated attack is almost a sure thing.
When using a Warthog against a Mantis try not to get too close. Keep moving constantly, don't roll the hog near the Mantis or get hung up on a rock. Constant fire with the turret, (paced to avoid overheating), will take the Mantis down in about 15 or 20 seconds. Team fire will increase the likelyhood of success.
Using a Banshee against the Mantis is not recommended unless it's part of a bigger plan. 1:1 vs. the Mantis the Banshee will almost always lose. Attacking a flaming/damaged Mantis with a Banshee might have better odds.
Mantis vs. Mantis battles haven't gone will for me yet unless I was able to surprise the enemy Mantis. As I described above use the turret along with your rockets to quickly overwhelm your enemy counterpart.
One key strategy against a Mantis is surprise - bringing a vehicle around a corner quicker than the Mantis can turn on you will give you a head start in taking it down. Use callouts to relay Mantis position to your teammates.
I tested a few weapons against an undamaged Mantis. Below is what it takes to kill a Mantis, (without team help) with each of these weapons.
Sniper Rifle - 8 Shots
Spartan Laser - 2 Shots
Rail Gun - 5 Shots
Gauss Hog - 3 Shots
OK... I'm sure I missed something but this is certainly a topic that will be important to us. Ragnarok seems to be favored in voting more than the other BTB maps so far. Also, I'd be surprised if one of the future DLC maps doesn't include a Mantis. Feel free to critique my strategy, adding to it when we learn something new or correcting a mistake I made. I'd like to do some custom game or Forge testing to learn more soon.