You are hard to please.
Shade Mode is fucking awesome. I just finished playing a match on Hard too and it lasted pretty long. The enemies take long to beat, the Marked Man in particular takes the longest and it's awesome because it forces everyone to move around the whole map trying to hunt him down. The instant exploding grenades are great too. For my first match on Hard difficulty, I was actually challenged unlike with regular co-op arena on Hard it just bores the shit out of me. I'm going to wait for my friend to get on so we can try Crushing....I don't even know if I'll be able to handle that just yet!
So far, Shades Mode is fun.
What is different versus normal co-op arena?
The enemy can teleport. Ok.
Also, sometimes one of the enemies has a target on his head and you have to kill him. Btw, you dont have to hunt him down. He comes for you. Its not like he runs away, or his AI is different in any way, lol. I'm not kidding. Stay in one area. He'll come to you. 75% of the time, the marked target spawned a few feet away from us.
I would have done things differently.
1. Make it longer. 8 rounds? Really? What is the purpose of that? Make it much longer, make the enemies more aggressive as it goes, each 5th round, throw in a special shade that has one of those perks like unlimited ammo, pak-80, etc.
2. Different enemy types. Why no guardians? Those guys would be way more fun to fight. High agility/mobility, but lack of any long range weapons. Have them as their own round, or mix them up with the shades.
3. Secondary objectives. Sometimes secondary objectives pop up, and if you complete them, your team gets +2 (3, 4?) bonus medals. It could be something simple like deliver the idol, or 3 areas pop up simultaneously. Each party member has to stand in each area and capture it at the exact same time. Or an object appears somewhere on the map that can only be destroyed by melee. Shooting it does nothing. If you destroy it, then bonus medals. It presents a risk/reward type system. You dont have to do them, but if you do, it could give you that edge for the later rounds.
4. Every 10th round, spiders. Except these spiders cant be killed at all. For a few minutes, you just have to survive. Run like fucking hell. Climb everything you can, jump, try to outmaneuver them. Shooting them slows them down though, and grenades stun them.
These are just some ideas I have come up with from the top of my head right now. This is what I would expect from new content. Not slap together some shades in co-op arena and call it a day.