Earn enough points using Shield Boost to cross the points threshold and this challenge will be completed. Any difficulty, enemy, or map will count towards this challenge.
Shield Boost is unique because it isn't an offensive skill. XP is only awarded for restoring the damaged shields of teammates by using this skill. The teammates must have shield damage so that shields can be restored or no points are awarded.
Friendly constructs like an Assault Turret or a Remnant VI can also be shield boosted as many times as possible and will always give the standard XP reward even if they aren't damaged.
Trying to damage your own shields and then using shield boost doesn't award XP. This must be allies or constructs to get the XP reward.
Using Shield Boost only awards +40 Support for each successful use, so it takes a long time to get to 45,000 points. This +40 XP is the same no matter the difficulty level.
It is possible to take the Angara Insurgent with Shield Boost and Assault Turret into the Tutorial mission to get easy XP rewards for this challenge. Throw the assault turret and then stand nearby hitting shield boost for the XP reward. While it may take some time to get the 45,000 points, there are no enemies in Wave 1, and you are only limited by the recharge speed of the skill. Using equipment and boosters to reduce recharge time will make this grind take much less time.