In terms of merit, it should not be normal for the user to "relax" and not earn more merits.
It shouldn't but that's exactly what can happen if you do. Apply that in every segment in life. If you don't do your best, you can't expect the best results and/or the greatest rewards. Those who work harder, will reap more benefits.
I want to say, does it have to be a battle for every merit or should it still be possible to get them with a relaxed attitude towards it?
You are not going to get them in the same quantity if you stop caring what you write and how. The content might still be merit-worthy to some degree but not like if you really applied yourself.
Should Legendary users submit their posts in various prize "unmerited post to be merited" contests, or create an endless "overview of XYZ" threads, which was able to help in getting merit?
Everyone does what they believe they should. If legendary users stopped writing quality threads and posts, I don't think low-level account holders will do it instead. The forum would then be at a loss regardless of the motives behind those posts.