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

I'm having a lot of trouble battling against shadow Lapras in Pokemon GO. It always takes me more than one Pokemon to beat shadow Lapras, so it is extremely frustrating and feels impossible to beat the Rocket Leader Sierra.

What are the best counters to shadow Lapras 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.

When it comes to rocket and leader battles, pokemon with fast charging charge attacks outshine all others.

Rocket NPCs stop attacking for a second or two whenever either person swaps pokemon or unleashes a charged attack. Use these short periods of time to your advantage! With the right fast attack and charge attack, Sierra can only get 1-2 fast attacks in before you're ready to use another. This works even when you're at a type disadvantage.

If the Lapras has a water fast attack, a Giratina-A with Shadow Claw and Dragon Claw would work very well. If the Lapras has an ice fast attack, a Magnezone with Mirror shot works great (it's neutral damage, but it also has a chance to lower Lapras's attack).

Check out this list for a list of which pokemon have the fastest charging attacks, and plan your matchup with Lapras accordingly. Good luck!