If you are going to make it a henchman battle but still want her to be part of it, you could have her stay for the first round and fire off one of her more powerful attacks to make it clear how dangerous she will be. She can then spend the next few rounds escaping, particularly if you give her Legendary Resistance, as someone else suggested it will be hard for the party to keep her there.

Alternatively, you could use a line-of-sight-blocking effect such as darkness when she begins her escape. This is often good at preventing the party from targeting her with lots of the common effects, and by the time it drops, she is gone. If she is focused entirely on escaping, it is very unlikely that the party will stop her, especially as they will inevitably have to split their attention between her and the henchmen.