There is no nerf needed for Field Aid but adjustments of players.
If you face a single hero leftover (like Rigard), don’t attack with a highly stacked team or with many healers. You need maximum 1 healer as backup for very poor starting board. Bring a rainbow team (or 2-1-1-1 team) so you nearly always find combinations. Charge as many heroes as possible and fire them all at once. The best time for it is the round after Healing Aid (and the heroes healing). Even here are limitations of course. That is the moment when you have to dodge the fight and let someone else to do the cleanup. Develop your roster and after 2-4 weeks that is not a topic anymore.
Healing Aid is a strong component, especially for midlevel where you have a lot of 4* healers. When you set up your team to face a healer defense, don’t rely on tile damage - the combined healing will give you headache even with an over-average board. Set up your team skill oriented: 1-2 healers, 1 supporter (def down or attack up preferred) and 2-3 hitters. Notice that you need to charge at least 2 hitters (better 3) to kill one healer from 100% down to 0%. Repeat until no defenders are left. You will not win all fights - but it would be boring if you would.
There are several guides for war strategies and even more guides about raiding, simply search in the forum. I simply offer my guide as reference: Yo Momma’s Alliance War Guide + Team Planner