I have no idea where you might have gotten that bit about Evil Druid armor increasing Occult Impact. That's not true and doesn't make sense, if you look at what evil druid card does.
Your standard +xx% race/size/element/dmg cards increases OI damage. OI is also based on your atk, so any +atk stuff works. +str cards have priority over +atk cards if it will help you reach your next str bonus. Otherwise, use +atk cards.
OI is "forced neutral", but it still follows elemental reductions - raydric cards, valk shield elemental resistances, etc. So for example, if your target is wearing raydric card, using cursed water allows you to ignore his 20% resistance against neutral attacks. However, if he is using Valk shield, using cursed water means the 20% resistance against shadow attacks from valk shield will work against you.
Aside from that odd thing about elemental reductions, being forced neutral means it will always be neutral with regards to armor element. This means ghostring card will always reduce your damage even if you use cursed water or endows.