Microsoft Ultimate Word Games

The Expert title is obtained by reaching level 80 in Wordament. You get experience from daily challenges, adventure map levels, quickplay, and multiplayer. Since this is one of the longer achievements to unlock, you should do all other achievements related to Wordament first since they'll all give you experience. Once you're done with everything else, you'll probably only be in the 30s or so, depending on how many adventure maps you do in Wordament. That means you'll be in for a pretty decent grind up to level 80.

The fastest way to level up in Wordament, by far, is playing a quickplay game on easy difficulty. Experience is awarded for each star earned, so difficulty does not impact experience earned in any way. This means you can start up a quickplay game on easy and very quickly beat it to get the experience, then repeat until you reach level 80. I know it will seem like adventure maps are the way to go, since they have the chests that give bonus experience, but adventure map levels are way slower than easy quickplay sessions, especially as you progress farther into the map. To put it in perspective, my fastest easy quickplay session is 6 seconds for all three stars. Realistically it should only take around 15-20 seconds per game to get all three stars. This allows you to level up very quickly.

One other thing to consider is upgrading to Premium. For $2, you get double experience for a month. I highly recommend going Premium, as it will make the grind in Wordament, and the other two game modes, only take half the time. In my opinion it is well worth it.

Upon reaching level 80, your title will change to "Wordament Expert" and the achievement will unlock.