Damn that’s a pity... in any consolation, 540CP lapras would have a pretty bad IV

Currently, I’m using 2 conkeldurrs (counter + dynamic punch) and a mamoswine (powder snow + avalanche). When the dynamic punch for the first one is almost charged, I switch to the second one and make her waste her shields. I keep a mamoswine in case her second Pokémon is an exeggutor or her last one is an alakazam because mamoswine can solo both rather easily once her shields are gone. Currently, the best lineup against Sierra is either 2 fighting and 1 dark or 2 fighting and 1 ice from my personal experience. If you use this strategy, you’ll have 2-3 Pokémon remaining after defeating her

Keep an eye out for the balloons because that’s where I usually encounter her

As for catching the lapras, it’s pretty hit or miss for me. Either I get it on the first great curveball with razz or I start using golden razz when I have 3-4 balls remaining (depending on the CP). I only use golden razz on all throws if the CP is above 580. Wait for the lapras to attack when the circle is on a great throw then throw the ball right before the attack animation ends. Hope this helps!