You can possibly lock your opponent's monster zone if he/she has no tribute monsters. Use Floodgate Trap Hole when your opponent Summons a Monster. Then disable the 2 remaining zones by using Ojama Knight's effect. You can then wait to get the cards that you needed or burn your opponent.
The text "monster(s)" on Floodgate Trap Hole means that it can affect multiple cards. See these screenshots below for example.
Had no clue this card couldn't be activated during the battle phase. Lost two duels because no where does it say on the card that it can't be activated during the battle phase. Couldn't flood a dreadmaster in lvl 30 aster duel, actually annoying.
it be activated during the battle phase but only in attack declaration and battle steps and not the damage step/damage calculation, every other normal trap follow the same rule except ATK/Def changing cards or cards that specify the time of activation, like "at the start, before or after the damage step/calculation", this also goes for quick play spells and continues traps but not counter traps.
I believe you just missed the timing of activation as the card says "When" so if the monster you want to floodgate was summoned in a chain with its summoning being chain link 2 or higher you will miss the timing to activate floodgate.