I'm trying to get these two achievements:

Defeat Hostile Model 1544-C Defeat Mecharachnid

But, I don't see them in the battle coliseum. Is there something I have to do to trigger a random event for these opponents?

They were part of a plot from a few years ago, and are no longer available.

Ohh, I see. I thought they were still available because the two achievements are not part of the "retired" tab.

Here's a ">link to a related thread.

Thank you! :)