It can definitely be tricky. I’ve been interrupting for my group the last few weeks and it has a steep learning curve.
I can offer a few tips. Basically you cannot tab target Llothis when he is inside Olms. You have to get him targeted before the off tank stacks him, and be careful no to lose that lock. As far as regaining target lock after he ports, consider Olms’ tail to be an impenetrable wall. You need to be on the same side of the tail as Llothis or targeting will be very unreliable. Once you have him tab targeted you can move wherever, and aI recommend getting back to the tip of the tail so you are ready for the next port.
Llothis will never port directly inside Olms, but depending where he was last killed, he may reanimate within the dragon’s hit box. For these you can forget about crushing shock. You need to position yourself between Olms and Llothis, inside the hit box, then block so Llothis doesn’t knock you back and do a good old fashioned bash. Be sure that you don’t have anything else tab targeted or even the bash might miss. You can clear target by pressing the escape key. After you get the melee bash you need to sprint or roll away because he usually ports immediately, leaving poison on the ground.
Make sure your team knows to leave you the entire front center of the room to move around, near the tail. Then you can just kite Storm of the Heavens in a small circle or square and be ready for interrupts. I recommend shielding when his timer is at 10s, so you can start spamming crushing shock at 11s and not have to worry about casting shields. While you are learning, Iceheart and Phoenix can be incredibly helpful. Sometimes you will have to put yourself in danger to get an interrupt, or even take a death. It usually saves several lives if you get it though, since you will start to lose people if he gets off 2-3 ticks of channeled bolts.
All this being said, I don’t think a perfect interruptor exists. I got Immortal Redeemer on my interruptor, and anyone in my group will confirm that I miss interrupts frequently. There will be times you can’t stop him from getting out 1-2 bolts. It’s really just about minimizing these times, and advising your group to shield or dodge if you know you’re going to be late on an interrupt. I hope this helps, tag me if you have any other questions.
Edit: I’ll also mention that is you are a Sorcerer you can Negate the poison AoE that Llothis leaves behind when he ports. Just aim right in the middle. You won’t be able to Negate all of them, and trying would be a DPS loss, but if there’s one really in your way it is an option. And if you are having mobility issues, either from the group kiting in your zone, or from Exhaustive Charges and poison all over the place, it can help to slot something like Accelerate or Boundless Storm for Major Expedition, and consider using swiftt jewelry. You will have to move around more than just about anyone else in the trial (maybe less than the off-tank).