Sentinels are the big armored bombs, you must take out 10 in a row before the fly away. They have a health bar on the bottom of the screen and when you kill them they act like a bomb and wipe out all live enemies on the screen. This if done in order will be achieved before the day 5 trophy.
Boost hurts them and so does overdrive so use them to your advantage. Also helps if you have someone with you attacking it as it will die much faster.