Strong, the Brutish Thespian

If a player has fewer cards remaining in their library than the number of rad counters they have when the triggered ability resolves, they’ll mill as many cards as they can.

Any effects (such as proliferate) that interact with counters a player gets, has, or loses can interact with rad counters.

If multiple sources deal damage to Strong at the same time, most likely because multiple creatures blocked it, its enrage ability will trigger only once.

Rad counters are a kind of counter that a player may have. They’re not associated with any specific permanents.

The cards are milled all at once, which means abilities that trigger “whenever one or more nonland cards are milled” will trigger exactly once as long as at least one nonland card was milled.

Strong’s last ability changes what happens when the inherent triggered ability associated with having rad counters resolves. You’ll still mill cards equal to the number of rad counters you have, and you’ll still remove a rad counter from yourself for each nonland card you milled, but instead of losing life, you’ll gain 1 life for each nonland card you milled.