My raid team finally finished the Story Mode versions of the two raid instances in Shadow of Revan in TOR. We were stuck on the third boss of Temple of Sacrifice, the Underlurker, for several weeks. We finally beat it by having tank switch to a DPS character, as well as having geared up more.
It's interesting, because this fight fails for a very simple reason: target-switching is a lot harder for lower-skill groups than you would predict.
During Underlurker, three adds will spawn and do an AOE attack. After a few seconds, rocks will drop from the ceiling and the raid has to hide behind the rocks. Then there's a mechanic where the boss has a cross telegraph appear under him, and the raid needs to put a certain number of people in each arm of the cross (1 in front, 1 in back, 3 on each side) before it activates. This cycle repeats about 5 times before enrage.
When you first do the fight, you think the cross mechanic is the difficult part. But--save when it was very buggy upon first release--the cross isn't the hard part. The hard part is getting down the three adds while still dealing enough damage to the boss that you can kill it before enrage.
That is entirely about target-switching and bursting the adds down. Which is something that lower-skill groups have a lot of troubles with, resulting in a fight which is unusually difficult for its placement.
The best solution is to reduce the health of the adds and boss in Story Mode, to give more leeway for time lost to switching targets.