Using either "Xyz Unit" or "Xyz Territory" can help "C65" survive longer on the board and take down bigger enemies with its primary effect.
This card can lock an opponent whose deck relies on low-ATK Effect Monsters such as those which serve as Summon fodder (like the "Hunder" cards), or which act as defensive nuisances (like "Penguin Soldier" or "Metaion, the Timelord"). For players running against "Number C65," it is good to run a few protective Spells and Traps, or easily Summoned Beatstick monsters just in case you find yourself in such a bind.
Note that as far as "Number C" monsters go, "C65" is one of the weaker ones, both in terms of effect and base stats. Even its monster effect negation ability, which is powerful and unique among Xyz Monsters, is somewhat cheapened by the existence of more powerful options such as "Skill Drain" or "Fiendish Chain." As such, "C65" works best as a support monster, serving the deck best as a general hinderance or a lock-maker/breaker for pinch situations.
Given its summoning conditions, "C65" also works well in decks that run "Numbers" "96" and "43," (and their "Number C"counterparts) who have near-identical summoning conditions and serve as stronger "main focus monsters" for it to support.
If you aren't using "Rank-Up-Magic" cards, you can use this card in a "Crashbug" Deck, as the main trio can easily assemble the necessary three DARK monsters needed.
List of Level 3 DARK monsters that can be used as Xyz Material to Summon this card: