Fyi Most Slowbro (unless they don't have defence investment) can take a crunch from Mega Cario and Burn it with scald or (if you are dumb like me and run it) Focus Miss it into oblivion. But Mega Khan crushes them.
In my experience, Slowbro switches into the Nasty Plot and then dies to Dark Pulse. I run Rocky Helmet / Slack Off to assist against Mega-Khan sweeps. I guess Slowbro can run Assault vest to improve my odds vs Lucario... but that just shows that this `mon cannot handle both Mega-Khan and Mega-Luke at the same time.
Fire Blast/Scald/Ice Beam/Psychic has good coverage, but you might prefer Shadow Ball in the last slot for certain Ghosts.
IMO, go mixed with Psyshock. Tentacruel seems to switch into Slowbro a lot, and having a "physical defense" option tends to be quite useful in this metagame filled with Assault Vests.