Just remember that even with -1 init, the Slaaneshi lord should be striking at the same time as the GKGM, so chances are you can take him with you. The Blissgiver is more lethal in most cases than a force weapon (only exception being things that are immune to Instant Death) since any wound it gives inflicts instant death, rather than just having a chance of killing on just one wound.
Even if he slows your Slaaneshi lord down, that GM shouldn't live through the encounter
Even if he slows your Slaaneshi lord down, that GM shouldn't live through the encounter