This is one of the easier components for totemic armor to get. It is more a matter of patience and annoyance than anything else. Here is a suggested plan.
Tidal lords pop basically in two areas and you can alternate back and forth to get them. First, go to the aqua goblin camp and take them out (do the casters first as they heal the others). Most are about level 20-24. Single pulling can be difficult.
If there are no tidal lords, you should see them on your next round. While you are waiting, swim out to the island in the middle of Dagnor's and sweep the island looking for tidal lords. I usually see one but occasionally a second pops. Note that when you sit to med, you will be attacked by the diseased rats and the undertow skellies, so be prepared.
Once you sweep the island, go back to the camp and see if they have popped. If not, sweep again.
Just keep going back and forth between the island and the camp. Eventually, a tidal lord will pop that has the shell, but it may take you some time. These are rare.
Good luck!