This probably seems like a weird question, but can you make your sylesti lose a level, or can they by other means lose levels?

Nope. And yeah, weird question ")

No, they cannot lose a level. But if you want them to stop levelling up (usually for arena pets), you can use a philter of unlearning. If you use that, the pet will not gain experience even when you battle or use it for missions.

You can get the philter from the scale shop, or on the trade broker if other players sell it. To enable the pet to gain experience again, you can use the philter of learning, also from the scale shop.

Thanks!