I didn't have time to answer due to other stuff so I'll answer now:
Mega Slowbro is nowadays nearly unviable for any Boss Fight except a few due to the massive Slack Off PP nerf, although you can theoretically still do many bosses if you run Rest instead, it is incredibly risky due to the 2 turns you are forced to sleep for. Mega Slowbro is relegated to PvE Quests now.
Several problems here:
Unlike Mega Slowbro, Magic Guard Clefable is NOT immune to crits. Any crit and it's over. This strat requires you to use 6 Cosmic Power, several Softboiled (varies depending on which boss), many, MANY Charge Beams (which is a 50% chance for a SPATK increase, not 100, which can also miss, AND Ground Types are immune to it) without fainting. Softboiled also has its PP nerfed and you having to risk it every single turn. The chances are absurdly low.
The other issue is that the entire Dark Type is immune to Psychic, which means MANY bosses cannot be done, contrary to just running Surf or Ice Beam. It's the problem with running Stored Power instead of Power Trip sadly. Many bosses would straight up be impossible with that sweeper sadly.
EDIT: Tbh this guide is outdated after gen 9's implementation, I need to update many things.