Getting the tribute fodders can be easy if you use tokens or Ra's Disciple, but it won't be good if you don't pair them with other cards that can take advantage of it. You will also need some protection spell/trap cards in order to tribute summon for the next turn, or use Double Summon instead, making it quite a heavy investment and can be bad if your opponent can counter it.
Special summoning Slifer will only make it stay until the end phase, but it can still be effective if you summon it during your opponent's turn before they summon monsters. It can also still be good if you want an extra attack during your turn if you can secure the win.
Since Slifer can destroy, or cripple your opponent's summoned monsters, people will often set their monsters to defend against an attack. You can use cards like Meteorain or Big Bang shot to inflict some piercing damage when attacking them, or you can simply go for direct attacks instead. You can use cards like Shadowslayer, Secret Pass to the Treasures, or Shooting Star Bow - Ceal. Gravity Axe - Grarl can also be helpful to keep them locked and prevent flip effects.
Guardian treasure combined with infinite cards is a way to quickly power up slifer. Also mausoleum of the emperor can help you get high level monsters on the field to stall. And the creator can also bring back powerful cards from the graveyard to tribute
Where is a goood slifer deck including infinte cards I really want to know to farm out some duelists