My experience with Sierra is that I brought Darkrai, Machamp and Metagross only to see Houndoom killing Darkrai with Fire fang and Metagross standing, so I replaced the Metagross with Swampert.
She used Sneasel, Hypno and Houndoom; My tactic is to farm energy with the Machamp on Sneasel, use 2 rock slides on Hypno to bait all of her shields and kill the Hypno with Darkrai. I didn’t use Swampert however, as I farmed some energy on the Hypno, finished it off with Dark Pulse and the Houndoom was unable to kill it with Fire Fang. I killed it instantly in one Focus Blast.
In your situation I would had been just ran Machamp Raikou Darkrai. Machamp farming energy on Sneasel and baiting both shields on Lapras, Raikou finishing it off and if possible, fire Shadow Ball on Alakazam. If I cannot I kill it instantly with Darkrai.