Do you fight on very easy mode? It's super easy to lose a tournament.
Maybe you have top tier armour? Even then, a couple of headshots will get you on realistic.
Oh, maybe you're one of those people using the exploit where you run around them and they can't do anything? In which case, you're not playing the game as intended and are basically cheating.
Hah, read a bit more of your reply and yeah, you're using the exploit. Well there's your problem buddy. You're not finding them difficult because you're exploiting the enemy AI to make them stand still and let you beat them down. Try not cheating and you might find them a bit harder.
Unlike you, I'd rather not ruin the challenge of the game, and therefore the gameplay, by cheating.