Jesus Christ, I know how you feel. I've got Slifer as well, and I can never seem to beat him. But I do have some tips:
-Don't summon Slifer The Sky Dragon until you've discarded both Marik's Lava Golem's to the graveyard. - Make sure you have a Jinzo or Royal Decree on the field - Mirror wall and other trap cards like that still effect God cards - Infinity cards is useful as it gives Silfer a huge advantage over Ra (one time Slifer had 11,000 ATK points whereas Marik's Ra had 5000)
For other cards: -Make sure you have trap cards like Mirror wall, blast of the tribute; any trap cards that'll effect Ra (and hopefully use them before you use Jinzo) -Deck Destruction cards (Like Needleworm) are useful. I have one at the moment (and trying to fit atleast three in) so I can try and discard his powerful cards straight to the graveyard. - Disabling traps and magic cards. He has really annoying ones (like that one when it disables your cards randomly. Funny how 'random' always targets my God card...)
But so far, I'm still stuck, too. I can beat him about 50% of the time, just not with slifer.
Thanks, but I just beat him yesterday, I was overjoyed.
I thought I would never beat him.
Tips: - Have trap cards like mirror force. - When summoning Slifer, make sure you have at least 3 cards, otherwise, it will have very little power and it will be almost useless. - Try summoning powerful cards so they can resist Yami Marik's attacks until you summon Slifer.
This is all I can say you.
Thanks for all you said.
Currently playing: Yu-Gi-Oh Nightmare Troubadour. Play it!