@Stefl
Sephi's a pain in the ass until you get his Black Materia HP attack. My suggestion is to put Scintilla as his main HP move rather than Octo-Slash (Scintilla is the attack where he stabs forward ultra fast and then a cut of light appears on the opponent).
Scintilla tends to come way faster than Octo Slash, so if you stagger your opponent, you can imediatly counter with Scintilla. Also you should take advantage of his EX mode, because of his Heartless Angel special attack. Give yourself some distance between your opponent, and press R+Square to cast Heartless Angel, if you're succesfull, you'll reduce your opponent's BRV to 0 and it will not grow back ;). Afterwards just way for the enemy attack, or punish him with a ground BRV attack and quickly go for a Scintilla.
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
