It is a dual Water/Ice-type, which means, in a rare instance, that its weaknesses outweigh its resistances.
It has affinities to the following:
Weaknesses: Fighting, Rock, Electric, Grass
Resistances: Water, Ice
Lapras has a lot of weakness which is a rare case, however, it has a natural defense: half of the types it’s weak against are themselves weak to moves Lapras knows.
Like, grass types are weak against Ice moves, and Rock types are weak against Water moves. So, if you fight Lapras with Rock or Grass types, you will fail miserably.
How to Beat Lapras in Pokémon GO
So, the best option which you can go with is to employ a Fighting-type. The best choices of which being Lucario or Conkeldurr. Both of these Pokémon can use Counter for their Quick Move, while Lucario can utilize Aura Sphere and Conkeldurr Dynamic Punch.
Well, to be on the safer side, it’s best to go with Lucario, as Aura Sphere provides higher DPS than Dynamic Punch.
Just remember that Lapras is a big and sturdy Pokémon, and it won’t go down without a fight. Don’t try to take one of these things on solo, as it simply has too much health for one Pokémon to handle on its own.