For most of of these online achievements, they can be done in a shared mech in team mode, and all the players in that mech will get progress towards these achievements.
So instead of doing 20 matches altogether with each of the 12 mechs if you had 4 people, you'll only have to do 5 if they are all in the same shared mech. So just put all players in a shared mech, and add another mech on another team with another controller. The AI bots you can add might work as well, but I cannot currently confirm that.
I was able to get most of the specific mech attack achievements to work in a shared mech as well, so "Used MetaGeckon's rolling ability 30 times in online matches", or "As Vidar, trapped enemies 3 times in a single online match", etc. Even if only the main host player did the inputs the other person/people in the shared mech will unlock the achievements as well. The online attacks achievements that need an ultimate attack didn't seem to work though. We tried a few times with a couple of them, but they didn't unlock in a shared mech. Maybe we were doing them wrong, but I'm not sure.
Not sure a ton of people will be playing/boosting this game, but knowing this tip should cut down on online boosting time considerably.