You should definitely check out Tripsdrill when you are in that area in Germany. I love the Gerstlauer launched coaster "Karacho" there (
).
For Europa-Park, there are usually no discounts available at all (except in combination with hotel stays). Even at the official online shop K1ngdaKa88 mentioned you pay exactly the same as at the park directly.
Holiday Park on the other hand is often working together with local partners to offer vouchers like "pay for one person and another person's entry is free". But it's kinda hard to find those specials when you are not from the area. Sometimes Holiday Park offers voucher codes for their online shop and posts them on Facebook (link). I would recommend to check out their page a few days before your journey and maybe ask on their wall if anyone knows something about vouchers from local partners. Anyway, enjoy Expedition GeForce! It's a really great coaster.