[Saxon]

Yes, the kappas will get hurt, but they are the chaff in this fight. Try to position them a bit in front of the vanheres, so the first round of firing should target the kappas instead of the vanheres. It will only help a bit, but it will help buy some time for those water blessed units to get up close and personal.

With Air 9 and Water 9, you can forge the items you will need to take an army underwater. You just need to look at the manual and determine if you need Construction 2, 4 or 6.

Also, you mentioned you can sail over them. Can you outflank them somehow? I know, old fashioned earth tactics, but it might work. Take a cheap province on the far side of him and threaten from two sides? Units with glamour help this sort of trick... It would depend on the map.

Finally, and you probably already have this in mind, but make sure you are winning the dominion struggle. If I am correct, he is getting free spawns and it is based on dominion. Make sure he is opposed in this, if only raiding the provinces with temples. You don't get such a nice benifit, but you do want to limit his advantage.