Big guns, quite simple unless you play as Imperial Guard, but even then Ogryns exist, or you can be luckey and get your Lascannon/Cannon in reach of the enemy gun.
Two hits from rocket launcher (krak missiles) should do the job. Your Chimera Lascannon has limited range, I'm not sure if traitor Lascannon works the same, but if it does, you can always try to snipe it from afar with sniper lasguns, Sniper Rifle, or Longlas.
If there is some cover, you can try throwing Krak Grenades.
Grenade launcher with incendiary, or Krak. Both will eventually kill it, while your guys are covered.
Probably the most important thing to keep in mind is to have and use smoke nades. Big guns appear to have limited vision trough smoke, while you can safely snipe them even if particular soldier doesn't see it. Then you can use anything, even lasguns will eventually break the armor and kill it, but for that you have Heavy Bolters and such. Even bringing one sentinel with lascannon is enough to deal with all of them if smoke nades are used.
