Stuff
If an opposing team allowed you to sit that close to the front, direct firing and doing that much damage, then they deserve everything they got.
People, no amount of "balance" or changing of game mechanics will account for player stupidity, stubbornnes ("LOLOLOL I HAVE 100k CBILLS I MUST BUY ATLAS!!!11211!"), or a refusal to work as a team.
As for the situation described further up, with a Long Tom hidden under an Atlas AECM, so what? Are you honestly expecting me to believe one Long Tom in a valley could dominate and control an entire team of mechs on Clearcut? With all of it's ridges, new hills and line of sight issues going on?
There is nothing wrong with the Long Tom, just like there was nothing wrong with BAs. Players just need to adapt, and think rather than just buying the biggest mech they can afford and walking slowly in a straight line towards the enemy.
If you want to talk balance, one serious issue in my opinion was the buff on ACs vs air assets. They needed it, but imo are TOO good now, so much so that air just isn't a threat anymore, and air being the best counter to artillery....