"The First Monarch" can help Tribute Summon this card. Using its effect, discard a monster that can be Special Summoned from the Graveyard immediately after discard and declare the DIVINE Attribute to change the Attribute of "The First Monarch". Then simply Tribute both monsters.
Use "Reckless Greed" to get more ATK immediately. Since it is a Trap Card, it can be useful if your opponent attacks "Slifer".
This card could work well with "Koa'ki Meiru Drago". This card has a DIVINE Attribute, while most powerful monsters in the current metagame are either LIGHT or DARK, so the opponent will find it difficult to Summon anything to defend themselves.
This card combos beautifully with "Dark Simorgh". The opponent will be unable to Set cards, monsters included, forcing them to Summon face-up. If they Summon in Attack Position, their monster will lose 2000 ATK due to this card's effect.
Combo this card with "Burden of the Mighty" to destroy more powerful monsters that the opponent Summons.
"Maxx "C"" can easily allow "Slifer" to gain at least 1000 ATK during the turn that it is played, although if it is already on the field, it is unlikely that the opponent will swarm with anything due to its destructive effect, so it might be a great idea to play "Maxx "C"" a turn in advance to Summoning "Slifer".
If a lot of Draw Engine cards are been using to keep this card's ATK high, there is the possibility of losing by Decking out. Cards like "Recycle", "Outstanding Dog Marron", and "Sword of Deep-Seated" can help counter this as they return the card(s) in the Graveyard back to the Deck.
If not Decking out seems a bit important than keeping "Slifer" powerful for a turn, "Localized Tornado" can be used to recycle ALL of your cards back into the Deck which are not currently on the field or banished.
Another alternative which allows yourself to Summon a monster while recycling cards at the same time is Special Summoning "Exodius the Ultimate Forbidden Lord" by recycling all monsters in the grave back into the Deck/Extra Deck. This can effectively chain into Summoning a Rank 10 monster as necessary.
Combo "Light of Intervention" or "Dark Simorgh" with this card and "Final Attack Orders", so that all of your opponent's monsters are forced to Attack Position, and then the effect of "Slifer" will be applied.
Use "King Tiger Wanghu" and "Burden of the Mighty" to destroy any monster your opponent Summons.
Using "Card of Safe Return" along with "Manticore of Darkness" can allow yourself to draw without limit. This potentially can increase this card's ATK to become close to the Deck capacity x1000, or ~40,000 ATK.
The effect of "Morphing Jar" can help keep this card's ATK high.
The "Gadget" monsters and "Ultimate Offering" are also recommended, as they maintain hand advantage as they are Summoned, and is easy to Summon using "Ultimate Offering".