The ultimate weapon ability is the Engineers team skill, to guarantee than you get the team skill everytime equip a piece of armor that has Transport+ on it, that way you will start every match with the transport ability from the start. Now simply get to level 5 in game (sometimes you will get the ultimate weapon earlier) and you will get the skill everytime, without the transport+ there is a chance that you will not be given the ultimate weapon ability. You do not need to get any kills with it so as soon you you are in the robot just press start and force respawn.
I recommend having an accessory with shorten cooldown on it as this will reduce the amount of time you need to wait before you can use it again. You need 4 people in the open circles to start the ability
Thanks to lightsup55 for this information:
The Sacred Song+ buff would be the better option as Sacred Song grants more EXP than Transport, especially when all 8 circles are filled. Combine that with Team Skill Exp Up buff to grant even more EXP from the use of team skills.
Another thing to note in the solution:
The Ultimate Weapon+ buff has been added to the game (as of January 24, 2014). There are some items obtainable through Happy Cards that have this as a permanent buff (orange), but it is possible to obtain a movable version (white) on any Cleric armor with buff slots.