Well honestly, defeating a group of MAUs is very tough. If you are  soloing and near some guard towers, run toward them. Theres no way you can solo a group of maus. The ost effective tactics I have seen used against me were mobbing and mobbing with skill force users. You will need a good organized party of people lvl 30 and above (Preferably mostly force users), and plenty of pots. The key to defeating the group of MAUs is to scatter them. When MAUs attack one target together, it is a sure kill. So you have to seperate them. The best way to do this is to attack different maus to lure them away from eachother and toward an ambush party or something. This is probably too organized to be used regularly. A more practical thing to do is to gang catapults first. Since they can not run, they are easy targets for range and force users. Once the catapults have run away or the pilots have bailed, focus on the goliaths. These are more dangerous since they can keep up with their boosters. The party is going to have to target one MAU and keep hitting it so it has no choice but to leave. Keep doing this till they get fed up. As long as you keep the MAU group unorganized and confused. They won?t get very far. Remember, you probably won?t be able to blow up the mau, but getting the driver to bail is the next best thing. Concentrate your fire on one mau at a time.

Edit: After playing NA/UK beta, I have to really stress that if you?re being chased by more than 2 melee mau, run. I?m a level 35 Accretian gunner with good gear and race leader status. This means that I get a 20% defense bonus + I have intense level 35 armor + a 25% defense bonus from being race leader. Even with all this, when I was netted and hit by 3 melee mau, my hp was brought down to around 100. Two of these mau had around 40 unit pt and the other one was pretty low. Considering all of this, if you see more than 2 melee mau chasing you, don?t bother trying to confront them. If they all target you, you?re dead.

Additional Info

MAU are constantly upgraded as the MAU players level up, all the way to Lv 48. This means their armor gets tougher and their damage increases and they can store more ammo. At Lv 40 or higher, the MAU players can access the Black MAU (or BMAU). This version has meaning looking graphics and blade arms (Goliaths) and vulcan arms (Catapult). MAU armor is tough against normal attacks, but it takes high damage from skill attacks (this includes melee skills, range skills, Accretia gunners in siege mode, and all offensive force skills).

MAU also have limited accuracy. If you manage to achieve high levels of Dodge/Avoidance, you can pretty much dance in the middle of a MAU party without getting hit, but be cautious because the Catapult debuff can reduce your dodge by -30 for 10 seconds.

Source: RF-Online-Wiki