@Jet Silver, your best option is Technomancer, since it has -50 All on one attack and boosts your damage nicely. And with tog boost, you will deal great damage, even when Seppy has low health and high all. And with Sepulchure leeching your mana away, Technomancer's power just increases. I recommend following equipment for you:
Ultimate Blinding Light of Destiny/DragonStaff of Destiny (BoA shown)
Necro Paragon Cape
Necro Paragon Helm
Unhallowed DeathKnight Circlet
Voidwoven Belt V
Seven "Lucky" Wishbones
Stats 200 in Str/Int, 200 in END and the rest 45 in CHA/WIS, whichever you think you need.
You should now have Immobility 80, All 24 and Evil 15, so Seppy shouldn't be able to stun you now.
And I assume you have trained Pet Dragon to max. I use my Drago with
149 in Protection
200 in Magic
1 in Fighting
150 in Assistance
100 in Mischief.
Start with tog boost and dragon shielding you. Then lower Seppy's all with "Event Horizon" and stun him with your Dragon. After that, you can stun him again with Drillbit, deal damage with Dragon, after that, use your Force-Sword/Photon Bow and lastly, stun him again with Sonic Boom Blaster. With luck, you have dealt 3000 damage to Seppy before he can even touch you.
Don't use your "Vent Heat" before the fight is near the end. When your Heat is 20, you can use Vent Heat to inflict +100 Health resistance, so Seppy won't heal when he has high all.