For the first mode, you choose which counters are removed as the ability resolves. If the permanent has fewer counters on it than the number of Elves you control, you’ll remove all of them.

A planeswalker with no loyalty counters on it is put into its owner’s graveyard.

For each mode, use the number of Elves you control as the enters-the-battlefield ability resolves, including Thornmantle Striker itself if it’s still under your control.