I'm a week late but like everyone else already said, comment, like things, make skins, posts, blogs, threads. Perhaps change your username so people don't think you are a bot, you'll get more interaction.

Happy leveling.

comment, talk, avoid begging for XP as its against the rules(you can get a warning)

Manage to get to level four for some reason or another

If you mean xp for raising your member profile level then put a comment in every map posted and a comment in every discussion. Even the biggest idiot will reach level 50 within a month but he will still be an idiot

I suppose you mean xp when playing a map

Most maps give you at least one tree, one lava bucket and one water bucket. That is more than sufficient to get unlimited experience. Lava and water gives you unlimited cobblestone. Tree gives you unlimited wood. make charcoal in furnace for unlimited experience. Convert cobblestone to stone in furnace for unlimited experience. Convert stone to smooth stone for unlimited experience. So you need a lot of furnaces to continuously gain experience. Use the final smooth stones to build all the future structures you require

I dunno, I kinda just made some random shit and said some things on the forums, now I'm level 28.

Create over 9999 accounts and use them to like your publications and posts

make an enderman farm in the end (they give alot of XP) you will need an endermite and a mminecart

Well… the best way is to post something[​s] really cool/useful that very large numbers of people view & DL & diamond (and maybe even – dare to dream – comment on )... but that's a rather tall order (and not something I've figured out how to do ☹️ )

More reliable (if slower) is to 'talk' to the other people on the site.
You've made something of a start to this in replying to some of the various "What is …" format threads, but those alone are limiting.
If nothing else one can always brose through various content leaving messages for anything that particularly strikes one's fancy [​assuming comments haven't been locked] — this not only gets you the XP, but provides valuable feedback to the various contnet creators.

Uploading a really big map