How to boost:
Siege battle has two teams (Alpha and Bravo) with 2 rounds, 1 to attack and 1 to defend (each). The team who scores the most points during their attack round wins.
The max room size is 8, but you can pick teams, so if you have 2-6 players 1 player picks Bravo team and everyone else is Alpha (if you have 7 or 8 put 2 on Bravo team).
Team Alpha will be attacking, so destroy a few targets to get some points (you don't need many) and then start crashing. There is a respawn penalty. The rounds are 20 minutes each, but if your whole team keeps crashing it shortens the match time considerably. (Note: Everyone should be using the CFA-44 plane as it has the highest respawn penalty at 39 seconds).
Team Bravo does nothing waiting for the clock to run down.
Team Alpha does nothing.
Team Bravo does not destroy any targets, and just crashes until the match ends.
With Bravo team not having scored any points the win goes to Alpha team. Rinse and repeat. With 4 people in a match (1v3) you can get one done in about 10 minutes. The more people the faster it goes, but yes this one is a bit of a grind.
As mentioned by ZZBroncos:
** You can do ranked or player matches for this, they both count
*** You must MANUALLY SAVE you game before you quit or your progress will be lost, the game does not automatically save multiplayer stats