Tardigrade needs that third spot. Cassowary and Elephant both massively over rated. Imagine a Tardigrade of equal size. No contest. It would simply SPEAR it's dagger like teeth into the target. Unlike all other competitors which could only fight in a location of their choosing, the Tardigrade...
I don't think aggression is hardness. If anything, aggression shows signs of bullying. Bullying is not hard.
The Tardigrade, pound for pound would win in a fight against any of the other competitors. In any climate. Let's say round 1 is in the hottest dessert. Round 2 in the coldest mountain...
You can't beat what you can't hit.
Let's say all of these animals are entered into a 'Hunger Games' style competition, with a series of extreme environments testing their toughness. There is only one winner.
#TeamTardigrade