First, banish a Spell Card in your hand to Special Summon "Monoceros."

Then, Normal Summon a Level 2 Beast-Type Tuner Monster and Synchro Summon "Thunder Unicorn." This will cause the effect of "Monoceros" to activate, Special Summoning the Tuner you just used for the Synchro Summon!

This means you will have 2 monsters on the field, allowing you to decrease your opponent's monster's ATK by 1000 with the effect of "Thunder Unicorn"!