So if you have a health problem, you do a Sun square to improve your health. Therefore, you are targeting the problem. Target the problem with a specific working for that problem, versus doing a Sun square to nullify your problematic Neptune, and hoping it will just happen to increase your health.
Does a Sun square remove delusion? Sun is light, light clarifies, so in theory, perhaps it will. But it would need to be programmed directly, not just to "offset negative effects of Neptune". We have members who have come up with experiments before that sounded way too indirect and unrealistic, which is why I always say to go for the problem directly. (Although, many beneficial and interesting effects happen from doing a square for one thing, and getting other benefits too.)
And there is more to delusions than what Sun will help with. Saturn, for example, but I don't advise doing its square to remove delusions. Having an open third eye, to see through spells and understand they are not real, will help.
What removes delusions? Understanding reality, and understanding yourself. So perhaps it would be best to work on that, instead of a indirect route mentioned above.