Others have touched on it.. It is all about sharing armor. It is fine to be a long range/sniping mech, But if you don't help with sharing armor, Or putting yourself in harms way in a push, then in the end you are only doing half of what you could to help the team. Two parts to this game are getting shot at, and shooting at the enemy.
So for example, an LRM, or long range assault, could actually lead out first, with a push, or when a push comes, and take some shots. This frees up damage that a medium or heavy does not have to take. After doing this a bit, Retreat.
Don't do this by peaking around corners one by one.. (this one drives me frigging crazy!!!) when a couple mechs are around a corner and people walk up, shoot, and then back up immediately, blocking the way, and blocking shots from the second row.. ect.. Doing that on a firing line is a sure way to die.
Say an assault, with a medium or heavy brawler and something else.. Is standing at a corner, If you all just peak around and take a shot, the 3 on the other side has the advantage, and focuses and kills you..
Now, if you all walk around the corner at once.. Say assault first, assault stays tight, the heavy should go next pushing on the outside shoulder, while the medium goes last and makes a straffing run to take other fire, Push um hard and brawl um from the flank/behind.. Any time mechs turn that is a good thing. good pushed and directed fire are the way to win battles often, Good snipers, and long range mechs still need to use up some of their armor early.. Snipe, then move up, take some hits, loose 30% of your armor, help a fellow out. Then move back and go back to your tasks.. You can tank at long range on many maps. But again it has to do with, you tanking, and the team firing at what is firing at you.
But again, that senario could of been walking around a corner at 600m, and medium is running like crazy using terrain for cover to flank, firing shots along the way. The medium gets to get close, because the enemy is firing on the assault out in the open, plugging away. Hopefully that brawler can get close by the time you are then dropping back, or taking some cover as some LRM's come, or even just long range shots.
Call targets, communicate, ect.. PUG life, this is an entire crap shoot, and most people don't even bother to talk until they died and complain about everyone not being mid readers.. Call shots, call locations, give info.. If your on a group that doesn't listen/react, i typically stop caring, if it all goes down into a big flaming pile because 5 mechs rushed behind everyone, after ya said, incoming from e5.. ohh well.
I think ya get the idea.. Basically ending the match at 100 damage in anything but a light.. and even they should of taken some fire.. good light pilots always do.. Rotating lines is another way.. A group of 4 pushes around a corner, then they all reverse, and another group takes it';s place,,
but again, this all takes communication,, and best place for that is not pugs, it's in a unit. There are lots of options for this.. Look for one that suits what you are looking for. Or just deal with Pug life, you will hear nerd rage from time to time..