1) Use the ignore feature! Click the little 'Ignore' button and ignore anything without a kanji reading, and if you like, ignore stuff you definitely know like 食べる or whatever. It saves time in the long run.
2) If you already know the readings or have a good familiarity with most of them, you don't want to waste time 'planting' things (= inputting the answer 6 times for each reading).
Fortunately Memrise has a hidden 'auto grow' feature that will fast forward you to the watering stage. If you right click the 'Plant' button and choose 'Copy Link Location' (or similar) you get a URL like this:
Change the grow/ to auto_grow like so:
Each time you visit this URL, 5 kanji from that part of the course will be moved to the watering stage immediately. I like to copy the URL and paste it into a bunch of tabs (on Firefox: Ctrl+T, Ctrl+V, Enter, repeat...) which will quickly grow all of the kanji in that part of the course.
Hope this helps!