^^ You can usualy find some good deals on dutch/belgium sites, like this one for example 37,50€:
http://www.funticket.be/index-nl.aspx
They might do some location checks. I didn't test it myself.
It should also be noted, that the park is running its "pay once, enjoy twice" campaign (Link) in April, which gives you a free second ticket to use at a later day except in July and August. Considering that you seem to usually aim for empty parks this shouldn't be a problem for you anyway. Make sure you keep your ticket after entering and get your second free ticket at Kaiserplatz (Berlin Plaza) from 12am or at the exits after 2pm.
So whether you get a discounted ticket or not, you would be kinda saving money in April anway.
Although in german, it's always a good idea to check this german fan forum thread for links to deals: https://www.phantafriends.de/topic/264-phantasialand-verg%C3%BCnstigte-karten-sonderaktionen/?page=39