Counter-heroes in duel mode (3.1).

It's quite possible. Here's an example: I might not be playing very well, but I played against an opponent who was much weaker than me in version 1.6. We played about 9 games, and I won about seven of them. We chose heroes randomly, and both of us got Nimues. In general, each of us had fourteen units (due to summoning), a big battle ensued, and he was left with just a few units. Then, this clever player simply eliminated my succubus, which was the only unsummoned unit I had left.
Of course, this was mostly my own stupidity, but the conditions were equal, and I was playing better.