Three snipers will appear on the ledge of the arena whenever Bo Zarran says Youll like this trick. Been working on it a while. You must kill these snipers ASAP, before these guys build a turret. If that turret is allowed to built, it will snipe players for 20k damage in rapid succession, wiping your raid.
Firebug
The person targeted by Mercenary Firebug must move to the side of the arena, away from the raid. The Firebug will drop a flame turret on that person, engulfing the area in flames.
Knockback/Cleave
Tanks should face Trasher against a wall, but not against a gate! Sometimes Trashers knockback can bug out, pushing the tank inside the gate and causing Thrasher to reset!
Stomp
This seems to be a proximity based AoE from Thrasher that hits for ~3k. Not frequent enough to be of a concern.
Bo Zarran
You dont fight him right away but he is nothing other than a named trash mob with a bunch of HP.
Thrasher is a really simple Tank n Spank boss with some added mechanics to make the fight interesting.
Tanking
Thrasher should be tanked against a wall due to his charge/knockback ability (he may also cleave). However, do not tank Thrasher against the gate as Thrasher can bug out when he charges and get locked inside the gate along with your tank. As funny as this sounds, Thrasher will immediately reset when this happens and your tank cannot get out.
Snipers
Throughout the fight, you will hear Bo Zarran say Youll like this trick. Been working on it a while. That is an audio cue for three snipers that will spawn in a group along the walls (location seems to be random). These guys have 22k HP each and dont hit hard. You can leap to them or grapple them down for melee DPS to kill them. While these guys are mostly a pushover, they can build a turret if not killed quick enough. This turret, once built, will basically target random raid members and hit them for 20k damage over and over, causing a raid wipe.
One interesting thing to note is that there are health thingies you commonly see in warzones along the ledges where the snipers spawn. This allow melee characters who can leap up there to grab them while killing the snipers.
Tip: Use your minimap to look for red dots when you hear the audio cue. This will allow you to quickly locate them.
Firebug
Other than snipers, one random member in the raid will get targeted by a Mercenary Firebug. This person will have a blue circle underneath them, which is very easy to see. They will need to run away from the raid, onto the side of the arena. The Mercenary Firebug will run to them and drop a flame turret on them, engulfing the area in flames. These flames dont cover a huge area but they dont disappear.
Bo Zarran
After you defeat Thrasher and open the gates, you get to fight Bo Zarran himself. He is guarded by two Mercenary Heavy Gunner, both of which can be crowd controlled. Pull him to a corner and then basically tank and spank him. He doesnt do anything fancy and is a huge pusherover. Doesnt drop any loot either.