Garden/Graveyard OPS a bit too easy

How do you unlock the difficulty "difficult" I only have easy, normal, hard, and crazy?

Also have you beaten a super bean wave or yeti wave on crazy? Because that should be when you post this. 

Sorry. It got lost in translation. I mean hard. Some waves are hard to beat, but mostly it feels like the overall difficulty could be more challenging.

On normal it is a walk in the park until a Super Bean wave appears. Never had any problems on normal besides that.

But i prefer and deliberately look for games on Hard, as that provides the actual challenge. On Hard you take a lot of damage - fast. And if you decide to go too far away from the team, chances are high you end up dead with no one being able to revive you.

So just don't play on Normal, go for Hard.