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If Undergrowth Champion is dealt damage from multiple sources at the same time (for example, if it's blocked by multiple creatures), its prevention effect will apply once to all that damage. You'll remove only one +1/+1 counter.

If damage that can't be prevented is dealt to Undergrowth Champion while it has a +1/+1 counter on it, you'll still remove a +1/+1 counter on it.

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

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.

Whenever a land you control enters, each landfall ability of the 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.).