Monstrous War-Leech's second ability is a replacement effect, not a triggered ability. It happens at the same time that Monstrous War-Leech enters the battlefield, it doesn't use the stack, and it can't be responded to.

If a card in a graveyard has an {X} in its mana cost, X is 0.

If Monstrous War-Leech was kicked, it enters the battlefield with power and toughness each equal to the highest mana value among cards in your graveyard, including cards that were milled due to its replacement effect. For example, if you have no cards in your graveyard as it enters and you mill three land cards and a card with mana value 4, Monstrous War-Leech is a 4/4 creature entering the battlefield.

The kicker ability doesn't let you pay a kicker cost more than once.

If you put a permanent with a kicker ability onto the battlefield without casting it, you can't kick it.

If a spell's kicker cost was paid, the spell is "kicked."

If a card or token enters as a copy of a permanent, the new permanent isn't kicked, even if the original was.

To determine a spell's total cost, start with the mana cost (or an alternative cost if another card's effect allows you to pay one instead), add any cost increases (such as kicker), then apply any cost reductions. The spell's mana value remains unchanged, no matter what the total cost to cast it was.

If you copy a kicked spell on the stack, the copy is also kicked. If the copied spell is a permanent spell, the token the copy of that spell becomes when it enters is also kicked.