I've heard of this LOS before.
It's the tactic where you run behind an object while the rest of your team runs ahead and gets killed, right?
Say a mage is casting frostbolt at you. Are you in the open? If so thats generally bad to begin with.
Close to an object though with decent reactions you can ruin their frostbolt.
You dont want to fight ranged DPS out in the open, you want to fight on your terms. As melee thats using objects to your advantage. Or as a healer. Do you stand in the open? Or do you stand by a box with your teammates just -barely- in LOS?
Constantly watch teammate and enemy positioning and adjust accordingly. PVP 101.
In regards to first kill target. Positioning is a hugely important factor. Strats can change on the fly if some enemy gets caught in the open. It might be all well and good to kill the spriest first in general, but what if he is kiting the crap out of you while the mage is unloading and polymorphing?
When being bursted, turtle up, go defensive and kite enemies around so your teammates can do the work. When your teammate is getting bursted switch roles. And understand when your enemy is doing this same thing to you. It all comes with practice though eventually you just know when to swap instinctively, but most of the swap calls will be because major cooldowns are blown or poor enemy positioning. Not even always their fault. Someone can play tight piilar game the whole match and position well, then a bad fear with trinket down sends them out in the open, quick pet nova roots them in place and you burst in 4 seconds flat.