Play on casual difficulty for this one (although you could get it on Normal or Hard, but gets significantly more difficult, especially on Hard.)
To get High Morale you need to perform well in combat.
Getting headshots increases morale the most, also the less damage you take the better.
Using Focus Fire (RB) with your squad also helps a ton on getting higher morale and will help with your 250 kills:
You will know if you have high morale as on the bottom left of the screen it will say "Squad Morale High"
You will get this relatively early in the game, especially if playing on Casual (I got it before finishing Episode 3)