Using Level 3 monsters with effects that activate in the Graveyard are ideal material monsters for this card. The best cards to use are "Shield Warrior" and "Necro Gardna", as they activate while already in the Graveyard.
Use the effect of "Lava Dragon" to Special Summon 2 Level 3 Dragon-Type monsters like "Masked Dragon" or "Hunter Dragon" from your Graveyard and use them as Xyz Materials for "Leviathan Dragon".
In a "Skill Drain" Deck, if this card has no Xyz Materials, then activate "Skill Drain". "Leviathan Dragon" will lose the ATK gain, but it will be able to attack your opponent directly.
If you DO choose to use the last Xyz Material of "Leviathan Dragon", have ways to prevent the opponent from going into Defense Position, and make sure they keep a monster on the field. Generally, "Leviathan Dragon" is less useful with no Xyz Materials, but in a defensive stall Deck, that uses cards like "Marshmallon", it would actually be preferable to use both materials.
Use "Numbers Overlay Boost" to recharge the Xyz Material of "Leviathan Dragon" for direct attacks or extra 1000 ATK boost.
"Space Cyclone" is a good counter for "Leviathan Dragon", especially after it uses it effect once.
For the best results with this card, overlay "Gagaga Magician" (declaring Level 3 with it's effect), and "Gagaga Girl", allowing you to use the effect of "Gagaga Girl" to make your opponent's monster's ATK 0, allowing you to deal massive damage to your opponent.