This card can be searcher|searched by Dark Sage, Ancient Gear Drill, "Angmarl the Fiendish Monarch", Left Arm Offering, Terraforming, An Owl of Luck, Demise of the Land, Ancient Fairy Dragon, Madolche Butlerusk, Planet Pathfinder, Pop-Up, "Set Rotation", and Warrior of Atlantis.
This card also works well with Level 5 or 7 WATER monsters, since they can be Normal Summoned with one less Tribute while this card is face-up. Giga Gagagigo and Levia-Dragon - Daedalus are good cards to use for this, as the former will have the most ATK you can Normal Summon using this effect, and the latter can perform a Field Clearing|field wipe after it's Summoned.
Works well with Infected Mail and a "Stall" Deck with a monster recycling engine.
This card and DNA Transplant can be used to upset the Levels of monsters players plan using to perform Synchro Summons or require them to use more monsters in the Synchro Summon.
* You can use Possessed Dark Soul in combination with the combo from before to take control of all of your opponent's monsters which became Level 3 or less due to this card.
* You can also mess with your opponent's Xyz Summoning with the above strategy.
This card combos well with Amulet of Ambition and Atlantean Pikeman. The Level of "Pikeman" will become 1 and it will have 1600 base ATK, allowing it to defeat just about any worry-worthy monster in battle. "Pikeman" will also be eligible for Opti-Camouflage Armor, allowing it to attack the opponent Direct Attack|directly.
This card works well against Burden of the Mighty.
This card makes it more difficult to Synchro Summon, which is particularly harmful if you use Brionac, Dragon of the Ice Barrier.
* Although in some cases it may make it harder to Synchro Summon, combining this with Grave of the Super Ancient Organism will prevent your opponent's Level 6 or higher Special Summoned monsters from attacking or activating their effects while letting your Water/Level 6 Synchro Monsters be able to attack and activate their effects. A perfect example is "Brionac" since it will become a Level 5 monster, evading the effect of "Grave of the Super Ancient Organism". Another good possibility is Dewloren, Tiger King of the Ice Barrier.
This card can be useful when using an "Atlantean" Deck that focuses less on Extra Deck support. Through the effect, Atlantean Dragoons will be able to attack directly with its own effect, while also letting Atlantean Marksman get through an opponents monster frontline to deal damage, allowing the easy Summon of Atlantean Attack Squad. If both "Marksman" and "Dragoons" are on the field, and "A Legendary Ocean" is present, it is likely that you will be able to deal over 6000 damage in a single turn.
* Also, thanks to the effect of "A Legendary Ocean," all of the "Atlantean" monsters (except for Poseidra, the Atlantean Dragon) will be able to use Aegis of the Ocean Dragon Lord, allowing for easier plays against Decks that would otherwise destroy the cards before any damage can be dealt.
This card can be useful in a "Nekroz" Deck, as it reduces the level of the Ritual Monsters in your hand, allowing them to circumvent the Level-specific restrictions many of them have. For example, Nekroz of Gungnir, a Level 7 Ritual Monster, prevents you from using a Level 7 monster to Summon it, but with this card reducing its level by 1, you can use Nekroz Kaleidoscope and send a Level 6 monster from your Extra Deck to cover the Summon with one card (or use a Level 9 monster to Summon "Nekroz of Gungnir" and Nekroz of Unicore, now a Level 3 monster, simultaneously).