But how do you get high Ampules from this event? I didn't saw them anywhere.

For the reroll, I'm triyng Dracaena. It's the best chance to get her. School girls should be available in their 30FG gacha later. But I didn't see a single Dracaena so far. I always noticed it whit rerolls, some girls are rarer than others (certainly based on your account ID).
Now, I don't know if I should try more on Nidhogg gacha and then St Paulia or if I go on St Paulia right now. She was my starter on Nutaku. She deserves some efforts.