im training a mixed aegislash and I want to know if HP water would be good considering it covers ground and fire weakness (other moves are shadow ball, sacred sword king shield iron head) and for what should I replace it?
HP Water is useless >.>
A neutral Shadow Ball does the same damage as a super effective HP Water (Shadow Ball - base 80, 120 with STAB and HP Water - Base 60, base 120 when super effective).
It's a waste of your moveslot really.
Try Shadow Sneak imo.
No, HP Water isn't a very good choice for it even though it covers two weaknesses.
The moves you have currently are very good for mixed Aegislash because they provide more coverage. If you wanted priority moves you can replace Sacred sword with Snadow sneak.
Source: Knowledge, using Aegislash a lot
Hope I helped.