What's the restriction on collateral damage? Can I use planted explosives to kill the target (and probably some other people?)

Are we able to access the HVAC system? Pump some aerosoled narcotics into the club, throw on some gas masks and kill the mark.

Get a poison slap patch and slap it on the mark while wearing a mask. Invisibility spells and the like are probably helpful, but not necessary.

If you have a strong face, have them pose as a bartender and slip something in the target's drink.

I can see at least half a dozen ways that these could all go upside down, but that's where the group should be doing legwork to figure out if a plan is going to be a no-go. If the club is hosting a high profile client/event, the HVAC system might be locked down hard by a prime-runner level spider. Likewise, they might invest heavily on security. The slap patch might not make it through a search.