This achievement requires that you place in the top 250 of a "Top Player" leaderboard. Currently there are only five of such leaderboards, all related to total score. They are for all time, today, this week, this month, and this year. It is likely that more types of leaderboard will be added in the future, for things such as top scoring snaps and average snap score. Some general tips to get in the top 250:
The daily leaderboard would be the most approachable, however, all but the all time leaderboard could be managed if you get cracking at the very beginning of the period (i.e. at the beginning of the week, or on January 1st), just after the leaderboard has reset.
Change your language. English is without question the most competitive language to play in. More obscure languages will likely be added to the game over time. For now, there is only Spanish, French, and Italian. Any language would have a smaller user base than English. Just by randomly merging letters together you should be able to break into the top 250. Especially if you play at a time when the countries that speak a specific language are sleeping. NOTE: You most likely need to have 100 games played in the language that you are reaching the top 250 of the leaderboard on. This requirement means that English is probably your best bet, despite how competitive it is.
Using a solver will help you earn points and climb the ranks faster. See the "Addict" achievement for gameplay specific tips.
Overall this achievement is not that bad if you try to time you play with the leaderboard resets and even switch to a different, less popular language.
The achievement may unlock at the end of the round after all of the requirements are met. However, for some folks, despite having met the requirements, "World Champ" will not pop. If you met the other requirements, but it doesn't unlock, just keep playing rounds and occasionally fully exiting the game. Upon loading in and connecting to the Wordament.net server, the achievement will eventually pop at the Main Menu.