Good counter to Minion in Dominion: Intrigue
As you have seen, Minion is a powerful card that, unlike many other Dominion cards, "combos" well with itself: if you draw two you can play one for cash and one to get a new hand, possibly getting another Minion, and attacking the opponent as well!

Minion is a card that's difficult to ignore. There are several ways you might consider to handle it:


Probably the clearest way to handle it, and the one that is always available, is to pursue Minions yourself. The Minions are powerful and every one you buy is one that your opponent or opponents cannot. If you choose this route, you usually want to go all-out to buy as many as you can. In a two-player game, getting 6 of the 10 Minions while the other player is stuck with 4 is a big deal.
Cursing and junking attacks are powerful counters to Minion. By filling your opponent's deck with space-wasting cards, they make it much less likely that the Minions will "power up" by showing up in the same hand. Sea Hag, Witch, Ambassador, Cultist, Marauder, of of course Mountebank are a few to look out for.
"Draw-up-to" effects can be an okay counter to Minion's discard, although they're not as effective as they are against, say, Militia. Still, Jack of all Trades is a powerful card in any board and probably worth considering if you're not going with Minions yourself.
Of the Reactions, Tunnel is powerful on any Minion board, whether you're the one playing the Minions or not--all those discarded Tunnels turn into Gold! Horse Traders is another powerful choice if you can spare room for the terminal action, since it neuters the attack and gives you a free card to boot. Moat and Lighthouse blunt the attack, but aren't super powerful in their own right; Lighthouse is a bit easier to fit into most decks. Secret Chamber looks attractive (save your favorite cards for the hand you're about to draw) but if Minions are flying around, its ability to discard junk for cash might be worth very little.