If you use weapons that shake your opponent (eg. PPC, ERPPC, SRM, SSRM, MRM) than stay at range. There's no advantage in using them when your opponent is 10 meters in front of you - he is gonna hit you anyway by just blind-shooting. Stay above 100 meters range to your opponents, or best: as far as you can while still doing damage.
LBX10 has really good range - it's something majority of people forget. Don't wait till very last moment - shoot as soon as you can.
MWLL Wiki has huge
library of weapons - out there you'll find out tons of informations unavailable in game. Only: be aware that values presented don't reflect 100% of actual damage - for weapons with big splash damage, eg. UAC20, your in-game damage may be few times greater!
Did you know that units differ in armor depending on variant? It's not noted in buy menu, but on the MWLL wiki you can find an armor value of each Mech. Try to memorize your favorites.
If you have to quit the game - send all your money to a teammate before.
Any Mech having LBX can be a real threat for ASF pilots. It requires lot of training, but you'll be able to score free kills easily and help your team in AA duty. One of coolest Mechs in this role is LBX 20 Shadowcat.
Be aware and know when to retreat. You don't have always to finish off your opponent. Very often it's better to leave him for your teammates, but survive yourself and repair.
If you are just starting your career it's best to find one role for yourself on battlefield and specialize in it. Over time though do progress - learn new roles to be flexible. Flexibility is the best way to achieve victory.
Don't be stubborn. If you read somewhere that certain tactic does work, and you think it doesn't - go in game and try it. Than again. And again. Till the point you'll master it. Both: You'll learn a new thing, and you won't push your point of view pointlessly on forum.

If you are going to play as dedicated battle armor - learn the terrain and find a spot where you can hide for long, as well as path of escape.