So this took me a while to figure out, thought I should share in case others had trouble.

In order to slide between any ogre's legs, but Bruz in particular for the mission challenge, you will only need to dodge, not an actual leg slide. Specifically you have to dodge right at him, and only when he attacks with the dodge prompt.

Keyboard default is SPACEBAR, and I find I often need to hit it twice for the dodge to execute properly.

I do not know the default dodge for other platforms, but you can look it up for yourself in the menu, guide, combat, dodging. Their discription for Vaulting specifically is what you'll be doing to slide under an ogre's legs.