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!