For me the great majority of mistakes in reviews are indeed typos. For that you either use a script (i.e. [OBSOLETE] Wanikani Override ("ignore answer button")) or you simply slow down and make sure to double check your input before submitting it.
Local Ice Climbers who reach the summit of level 60 usually share here their story with WaniKani, so you can see how they did handle the humongous number of incoming lessons and reviews.
I am nothing more than a beginner, but when it comes to mistakes that are not typos here what does the trick for me:
- if the mnemonic for a specific kanji doesn’t help you, try to make your own mnemonics. I find this particularly helpful when english is not your primary language;
- if this does not help either, use one of the example sentences as a reference, a title of a manga/anime you know ecc… this helps me when I get a kanji wrong so many times that I always get used to input the wrong answer (i.e. you see 三月 and you still are 50% unsure if it is さんがつ or さんげつ);
- if you mistake a kanji for some other kanji write both down many times, by doing so you’ll point out more easily in what the two differ; be sure to write them down with the proper stroke order.