I was on critters for a while, and it has some good things going for it. It's huge. The requirement to write a certain bare minimum of constructive feedback on a piece, for credits, was worthwhile. And one of the best writers in my IRL group is active there--so I think that speaks well for the site.
OP, I don't recall them assigning critiques. I thought you could pick? The Captain was also responsive to me through email, so you could try emailing him.
I left critters.org when I got my first batch of critiques *back* (after earning extra credits through critiquing others). My story was bad, but most of the critiques were helpful anyway. Except one, from a guy who insulted me top to bottom and then promoted his website as a way that I might learn to write.
That's the reason for the rules on the site. Because of that one guy, and the amazingly bad taste his critique left in my mouth, I left immediately. Think about it this way-- Here on AW, a harsh critique is public. Everyone sees it. On the other hand, a harsh (and bad) critique on critters isn't public, and I think the occasional bad egg really misbehaves as a result.
So-
1. Don't pick that story. If you have to, then -
2. You can easily say positive things like:
...a. It's clear you worked hard on this. (even if you don't believe that, trust me, they did.)
...b. Thank you for putting your work out for critique--It takes courage.
...c. You have a unique perspective and that's valuable.
3. For the negative things--be kind but generic. Examples:
...a. The strongest part for me was the characters, the weakest part was the action. I'd like to see more active verbs. You can find examples at (website.)
...b. I'm curious about x. I'd like to see more time spent on that.
...c. One of the things I struggle with is comma usage. It seems like we have this in common. The website (x) has been helpful. Take a look.
And you're done and you move on with your life.