Requires 3 Tributes to Normal Summon (cannot be Normal Set). This card's Normal Summon cannot be negated. When Normal Summoned, cards and effects cannot be activated. Once per turn, during the End Phase, if this card was Special Summoned: Send it to the Graveyard. This card gains 1000 ATK and DEF for each card in your hand. If a monster(s) is Normal or Special Summoned to your opponent's field in face-up Attack Position: That monster(s) loses 2000 ATK, then if its ATK has been reduced to 0 as a result, destroy it.
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.
For example, look at Exo's deck below. It'll successfully accomplish point 1 and 3: summoning and powering Slifer. However, it will encounter trouble if his opponent just destroys Slifer. The guy who uses Ojama should fare better, since it has a way to OTK and clearing the backrow... Although it will face a different problem of being inconsistent. Not belittling them, but it isn't easy.
Tribute summoned the first normally.
With 2 other monsters on your field, use soul exchange on the opponent monster
With the last monster in your field use 2 soul exchange
Summon Horakhty and win the game
Dude, 3 tributes is part of the effect, even if Slifer's level decreased to 4 you still need 3 tributes to summon him
This card is a lie. It says card effects cannot be activated. But I was in a match where a flip card took control of slifer. In addition in another match, a trap-card destroyed slifer when it was summoned. This card was not worth the time farming for. Pathetic
This is a card ruling situation. If you do not know card rulings for Yugioh then you are one of the many people who make this mistake. "Cards and effects cannot be activated when this card is summoned" mean you cannot respond to the summon. It's not at all what you thought. Long story short you cant Floodgate god cards.