What are the best counters to shadow Lapras / Sierra in Pokemon GO?

Some general points of advice for fighting Team GO Rocket bosses:

• They stop attacking for a few seconds if you switch Pokémon. This means if you start the battle with a Pokémon other than your intended lead (preferably one that can comfortably take a few hits) and then immediately switch, you can get a couple of free fast attacks off.
• They also stop attacking for a few seconds whenever a charge move is used. This means that Pokémon with a combination of a high-energy fast move and low-energy charge move are at a significant advantage.
• The bosses will always shield your first two charge moves, and moves that change your stats or your opponent's stats in PvP still apply that effect even if you hit a shield. Combined with the previous point, this makes Pokémon that know Power-Up Punch especially potent.

Here are a few examples of Pokémon that can work well against Lapras specifically:

• Lucario with Counter/Power-Up Punch. It doesn't take Lapras's Water Gun very well, so if you're facing one with that move make sure to use the switching trick to avoid taking any unnecessary damage. By the time you've taken down both of Lapras's shields, Counter will be doing heavy damage and you should hopefully be able to finish it off without getting KO'd.
• Melmetal with Thunder Shock/Rock Slide. Another super effective combination of high energy generation and low cost, it doesn't get Power-Up Punch's attack boosts but makes up for it with much better bulk.
• Poliwrath with Mud Shot/Power-Up Punch. Similar to Lucario but with the advantage of resisting both of Lapras's fast moves. Mud Shot doesn't do as much damage as Counter but it makes up for it with even higher energy generation to take advantage of the free attacks after every charge move.