The best cards to target for this card's effect are cards that the opponent cannot easily play again, such as a Fusion Monster, Synchro Monster, Xyz Monster or a Nomi monster. If you stack a Nomi monster, it will force the opponent into a dead draw and probably make them helpless to act during their next turn.
Especially if your opponent relies on their next draw
Make sure your opponent has at least 1 card on the field for you to return to their Deck, so you don't have to return any of your cards.
Use "Conscription" after you activate this card's effect to Special Summon your opponent's monster to your side of the field.
While you control "Destiny HERO - Diamond Dude" and another monster, you can Set a Normal Spell Card with a heavy cost (such as "Final Destiny"), Tribute Summon this to stack that Spell Card on top of your Deck, then mill it with the effect of "Diamond Dude" and activate it next turn.
This card also works well alongside "Phoenix Wing Wind Blast", a card with the same effect. After using the effect of "Raiza" to "stack" your opponent's card on top of their Deck, use "Wind Blast" on your opponent's End Phase, stacking another card in the process. If you Summon yet another copy of this card and/or activate more copies of "Wind Blast", your opponent will have to fight back without seeing new cards for several turns. If the cards they currently hold aren't able to deal with your "Monarchs" on the field, having to draw the same stuff over and over again will make them lose the Duel quickly.