We killed Shannox for the first time last night. We kept Riplimb 60ish yards away from Shannox. We attempted to get him caught in Crystal traps, but he tended to run over the Immo traps instead and gain immunity. We had Rageface accidentally trapped several times, but never once did Riplimb end up in a Crystal Prison. The 60 yards + slows were enough to drop stacks.

As soon as the spear emote goes out and the "landing spot" shows up, the Riplimb tank should move him away from it. You can get some good distance, another 15-20 yards before Riplimb actually goes to fetch it. That's extra distance he needs to travel to get the spear to Shannox. Once he went after the spear, I would constantly apply Concussive Shot to him. It was enough slow to reset stacks for both tanks.

The "Dogged Determination" buff (which raises Riplimb's slow immunity) doesn't seem to have a time limit on it. Instead it seems to fall off when it reaches 100%, but we didn't do extensive testing on that so I could be wrong. You don't want to apply any slows when he's just being tanked (so avoid automatic slows like Crippling Poison). Only use slows when he's fetching. On the other hand, if he's at 80%-ish immunity after fetching the spear and getting back to his tank, you can use some slows on him to push him to 100% so it resets and you have full slow power when he goes to fetch again.

All in all a fun fight, only actually took us 4 pulls to do it, whereas we've wiped several times on Beth and Rhyolith so far.