Or does he just have bad luck? Every time so far I met him on shard where there was third leader with us, he always lost very quickly and never stood chance against his opponents. Every time I wanted to ally himm, but before I got the chance, he was dead and I had to change the challenge of one big realm. It even forced me to surrender once, to that necromancer guy, who had three incredibly strong heroes thanks to this.
Because of this I've never fought his heroes or garrisons so I don't know if he is using some low level and weak units or what.

Probably, most incompetent. Some players even edit players.var to give him bigger balls (aggro variable)