mine77 has fixed diff even if not documented on their getting started page. Consult the readme of your mining software to see how to apply it.
With Claymore you add it behind the payment address, separated by a ".", e.g. paymentaddress.10000 or paymentaddress.4000...

But keep in mind to not use a too low diff or your IP will get blocked as this is considered a "low diff attack". Consult a post with screenshots I made a few pages back about how often shares should be submitted.

Indeed, all these cryptonote pools have a broken auto-diff and will quickly adjust the difficulty with every round up until it tops at 200,000 or something, where they will leave it forever. No matter on which port, i.e. at which starting diff you connect. Which is ridiculous for any kind of rig, let alone for people mining just with a single CPU or GPU.

EDIT: With a single GPU try 2000. Should work well.