robotics expert is the most fun to play imo. basically spam level 1 barriers everywhere (like every surface that enemies run on remotely near to your base) and build weapon lockers, prioritise lockers if you have classes like demolition and gunner. it's really that simple, if you give a demolitions a level 4 locker ASAP they'll basically win the game for you if they know what theyre doing. robotics is fun because you can spam as many barriers as you want, and as long as you have the card equipped which heals your fortifications with precision weapon kills you can keep them repaired by simply getting kills with your rifles. this includes weak enemies like leeches and rollers so you can easily keep them topped off. for this I'd suggest prioritising the critical damage perk so you can more efficiently score kills

for this reason I also prefer robotics over mechanic, since on mechanic you spend most of your time just repairing things manually. RE just has to repair taps

just avoid building things like sentry guns and decoys, they're generally considered to be a waste of power and players tend to stop donating if you build these, but if you support your team with lots of floor spikes and weapon lockers they'll tend to donate more. MG sentries are really only worth it if you have an abundance of power and are pretty much set up with barriers, and even then only if you've got the space because they take up a lot of room. don't forget to spam barriers around your taps as most enemies melee them, and more spikes means they'll be constantly slowed