Microsoft Ultimate Word Games

There is a good chance this will come naturally while earning your 100 stars in Wordament Adventure mode, especially if you play a level that has a star requirement to find a 12-letter word. That is almost guaranteed to be worth 45 points.

The other common way to easily get words worth 45 points or more is from the multi-letter tiles, such as INTER- or TRANS- tiles, which are worth a lot of points on their own. You'll definitely come across these while grinding to level 80 and in adventure map levels. These tiles are usually present in a grid with a long word possible with them, with as INTERNET, INTERNED, INTERVENE, TRANSMIT, TRANSMUTE, TRANSLATE, etc. Whenever you see one of these special tiles worth a lot of points, using them in a medium length word is often enough to earn 45 points.

If you're still struggling, you could use a Boggle Solver, where you input all the letters in the grid and it tells you all possible words.

As soon as you spell the word, the achievement will unlock.