Battle — Siege // Creature — Elf Warrior
Battle — Siege
(As a Siege enters, choose an opponent to protect it. You and others can attack it. When it's defeated, exile it, then cast it transformed.)
When this Siege enters, destroy target non-Elf creature an opponent controls with power X or less, where X is the number of lands you control.
Creature — Elf Warrior
Winnowing Forces's power and toughness are each equal to the number of lands you control.
This card uses a replacement effect ('as [it] enters' or 'enters with') to modify how the permanent enters. This is NOT a triggered ability — it can't be responded to, and effects like Panharmonicon or Torpor Orb don't affect it.
This card's ability triggers when it enters the battlefield, not when it's cast. Effects that double ETB triggers (like Panharmonicon) work here. Countering the spell prevents the creature from entering, so the ETB won't fire.