Warlander

This achievement is one you will get in time, especially if you mainly use the squire and actually use the blocking mechanic in fights, but in-case you're in a rush like I was... you can use the Squire and make sure you have the barrier ability equipped which is available at lvl 1. Simply rush to whichever lane has the most going on (Typically the middle lane) and try your best to stand in front of all of your teammates but not so much so that you're going to get melee attacked but rather to tank ranged attacks and activate your barrier ability as much and as often as possible. Whenever my barrier was down I kept my shield up and tried to walk into ranged attacks.

Whenever you successfully block a multiple of 100 Damage with your shield you will have a popup for 20 points for preventing damage.

Doing this I went from 30% to 100% in only 3 games.

As an added bonus, if you have the barrier ability up and stand behind a gate being attacked and you angle your shield through the gate you can get easy damage preventions that way as well.