Power of the Colossus

Do not run-up to the boss and then use the group finder.
You'll find lots of other people, but they'll be phased once they joined your group. This is not because of warmode on/off.
The reason for this is phasing: Blizzard wants to avoid people PUG-hopping to kill all rares. https://www.wowhead.com/news=319430/lfg-group-finder-world-questing-is-much-harder-due-to-phasing-changes-in-shadowl. Unfortunately, this makes this group quest insanely annoying.

Here's what you should do:
1. play with friends
2. Alternatively, you can create a PUG group, but make sure EVERYBODY goes to Ve'nari or to Oribos or any other rest/no-fight zone. This puts you in the same phase. Then travel with that group to the boss. If someone doesn't listen, let them run back to Ve'nari.... yes, it's annoying.

3. Preparation:
It's a good idea to kill the elite add in front of the boss. If someone gets knocked back during the boss fight, they may face-aggro this mob. It's a rather tough mob and can kill people quickly.

4. Tactics: Tank the boss so it faces away from the group, but be careful with knockbacks. Careful with the knockback: You can be knocked off the platform if the boss faces the open side. If the boss faces the side you can be knocked next to the platform. Most classes need a priest to grip them back up.And if the boss faces inward, you may be knocked far back. If you have the aggro, the boss may reset at this point.

Oh. And don't try to solo the boss unless you're a tankadin or blood dk. If you are, make sure you plan in a good amount of time to kill.
Oh2. and if the other faction tags the mob, it goes grey and won't drop the core for you.

After a couple frustrating runs with a LFG, phasing and constant deaths trying to reach this quest, we found out that YOU CAN NOT COMPLETE THIS IN A RAID GROUP. Once we switched to party, we got the item after killing the boss again.

I killed it but was knocked onto the ledge below during the fight. I couldn't get back up to loot it and had to kill myself and run back.
The corpse had despawned by the time I got there and I then discovered that the Colossal Power Core does not get mailed to you, so if this happens, you'll have to kill it again.