There are 264 floors in Tartarus split into "blocks" that you'll climb throughout the story of Persona 3 Reload. On each block, there are powerful enemies called Gatekeeper bosses. These Shadows are in place to test your strength and usually serve as a good indicator of whether you're ready or not for the month's Full Moon Operation. A challenging Gatekeeper boss that you'll run into in Persona 3 Reload is the Venomous Magus. At this point in P3R, you should have more teammates to choose from and more Personas at your disposal. Thus, Venomous Magus shouldn't prove too much of a challenge, but you'll want to bring someone who can target its weakness to make the fight a bit easier.