I don't know what people are doing when attacking with a Blissey...You need to maximize DPS since you're time constrained...
Assume you may also need to use all 6 attackers, go with things which will SE into Lapras like Jolteon (glass cannon so not ideal), MACHAMP (BEST COUNTER IMO), Dragonite (resist water/HP, great stats in general), you could also use a double rock golem, by HP would hurt...
Could be so cool to catch a guaranteed high IV Lvl 20 Lapras...
Edit: Wanted to add, honestly Machamp is a counter (SE) in nearly 5 of the top 6 most annoying defenders (Snorlax, Blissey, Lapras, Steelix, Umbreon...probably more)...
Can't wait to get my 1st TM And use it on my Bullet Punch/DP Lvl 20 Machamp (then again, my 15 ATT Champ has BP/Counter).