Stheno does not have much to offer.

I caught a Stheno unawares in Nusakan today... At first I found it strange that it had not been killed off, even when I came back to hit it again a few hours later. Now I understand. After hundreds of rounds of combat with the Stheno, I have achieved skill gains less than the briefest encounter with a Z-bot. It must be that the other, more experienced Z-hunters here in Nusakan will not waste their time with it.