To double Tifa Lockhart's power, it gets +X/+0, where X is Tifa Lockhart's power when the landfall ability resolves.

A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.

Whenever a land you control enters, each landfall ability of permanents you control will trigger. You can put them on the stack in any order. The last ability you put on the stack will be the first one to resolve. As a result, you can have those abilities resolve in the order of your choosing.

A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.