It's do-able, but you'll want to max out Bug Buzz's PP anyway for the legendaries that resist it AND have Pressure. If it turns out you can't beat them anyway, that's what the third member is for. The only options other than Taunt that Whimsicott could use are Encore (since Light Screen isn't necessary with Quiver Dance - could be useful though) or Tailwind (for your third member), but overall I'd say Taunt is the best option; it's the option I'm going to use on my Whimsicott anyway. You don't want to switch Butterfree in unless it's definitely safe, it won't survive even moderately powerful hits because it's so frail. It'd be Taunt>Switcheroo>Charm until Whimsicott dies/Taunt runs out>Taunt/Set up in most cases.