Very nice and great choice of scene to depict!
Physical description-wise, I think Cowl gets a good description early in RotCG
but I can't look it up for you 'cause my RotCG is in a box somewhere. I seem to recall he has some characteristic scars. I don't think he would look old, as he is a mage of considerable skill, plus he's Avowed, plus you have to stay in shape to be an assassin-mage!
Topper:
Quote
"The figure – a man by his height – was swathed in shades of green: cloaked, hooded, wearing a faded tunic and linen leggings above green dyed leather boots. A single long-knife, the weapon of choice among
Seven Cities warriors, was slung through a thin belt. The man’s hands, faintly grey in the afternoon light, glittered with rings, rings on every finger, above and below the knuckles
[...]
His face was long, the skin a lighter shade of grey, the eyes dark and strangely angled. He looked to be in his early thirties, though his hair was white."
and also he's pretty tall.
Though in his Tatterdemalion/Rag-man guise he may look quite different, beyond just the ragged silks for clothing.
Weapons-wise, both seem to use long-knives and Cowl I believe had throwing knives at some point.
Warrens-wise, Topper definitely favours Rashan/Kurald Galain so voids, waves and tendrils would certainly be the way to go.
Cowl's magic seems to be most heavily favouring a lot of moving in-and-out of warrens (Topper being equally capable of this, too, though), as well as being undetectable (visually, orally and by magic sensors) and being able to pierce through all sorts of defensive wards and barriers. You could show this by having one of his weapons glowing with magic as it cuts through a wave of magic or a ward of Topper's. Cowl also hurled some projectile attacks against Silanah, so he's certainly capable of that, too.
Setting-wise, the part that I really like about the Topper-Cowl duel is how Possum muses that they'll be constantly jumping in and out of all sorts of warrens as they fight - Claws and other assassins pop out of one warren to ambush all the time, but Cowl and Topper seem uniquely capable of constantly shifting warren every few seconds to try and get the drop on each other and it's a very neat idea. I think you should try to include that, if you can - something like Cowl stepping from warren A into warren B, and Topper from warren B into warren C, mid-fight and with open portals showing other worlds beyond. Might be really hard to do, but totally worth it...