Ugh yeah, I also hate 加える… even got it wrong again just now looking at it - constantly misreading it as though it’s 増える. My mnemonic about including koalas never comes to mind when I need it…
To OP: The Death levels were absolutely the worst for me too - the true slog begins and the Burn reviews begin showing up. Suddenly leech management becomes as important as learning new things. It’s a big change. The Hell levels that follow are just long. Death is the true test of one’s mettle.
Since leech management is important, Leech Tables are a gamechanger - suddenly all my struggle points were there to squint at every day. Grab from [Userscript] WaniKani Dashboard Leech Tables - Notice your leeches (this stopped working for me though…)
The other thing I started doing is pre-reviewing - I grab a bunch of Locked kanji/vocab off WKStats by literally copying and pasting the little blocks into a text editor where they come out comma-separated like 使,苦,試,部. Then I try to guess their readings and meanings before I have to review them. By around Level 20 with enough Japanese under your belt, you can start to make some pretty good guesses - moreso once you hit the Hell levels, but you can get a leg up with the Keisei script at [Userscript] Keisei 形声 Semantic-Phonetic Composition - anyway, reviewing stuff before it becomes a lesson means the horrible surprises are taken care of.
頑張って！
A quick update from Level 55: After a point, the leeches just built and built and built… and tracking them all became a timesink in and of itself. So for the last few levels with hundreds of leeches to juggle I’ve switched my review style to high throughput mode - review often, review quickly and where leeches are concerned: bring it on. If the right answer doesn’t come swiftly, that item probably wants another trip through Guru (or even Apprentice) anyway. You don’t want to start doing that toooo early on though, otherwise you’ll be juggling leeches for a long time.
The “Reality” levels are about as grindy as the Death levels too… but by then at least you’ve already been through the Death levels so you have a precedent! 