If I am remembering correctly the leaves are only drop from certain enemies in the forest (The one area with the fairy boss), time of day dosent matter for those. Night time is very optional for most things. But rare enemies can only appear at night, which can give tons of materials and exp.
Difficulty wise, it's hard to say anything very specific without seeing you stats, level, equiptment etc. But in genreral on normal most enemies shouldnt be able to one shot you, except you are underleveled or have equiptment with low defense stats. If you tell me the areas and enemies which cause you problems I probaly be able to give better help.
One tip for a few extra levels is: For each enemy you kill you get exp shards. Using them can give decent exp amounts if you have a bunch. In town at the witch shop you can even enhence them to better version.