I thought that I'd be losing the match when I ended up with one card left while the opponent had all his cards intact, so I skipped to results and to my surprise I won the match.
How in the world did I win the battle!?
I had to watch the replay a couple of time until I found the answer. It turns out that from the 20th round, a point will be deducted from the HP of all cards in the arena due to fatigue until there is a clear winner.
Glad I picked the summoner Zintar Mortalis.
Had I chosen otherwise, the match would have been over prior to the 20th round.