Minikit #1-3: The best way to get the four minikits in this area is simple: trample everything. Charge into every box you see. Some contain minikits, some need to be destroyed en masse to earn yourself a minikit. In any case, smash through everything, and you’ll get the four minikits here.
Minikit #4: Once you begin fighting the Witch King, use Gimli’s axe to shatter a cracked tile in the southwest part of the arena. A minikit is hiding underneath.
Minikit #5: On the right side of the field, use Sam’s spade on the patch of soil to attach a glowing handle to the nearby cart. Use the Uruk-hai to yank the cart, unloading an injured solider.
Dig up some soil just south of the cart to get a red herb. You’ll need this in a moment.
Across the field, you can dig up another soil patch for a yellow herb. Combine this with the red herb to make a First Aid Kit. Give this to the injured soldier from before to be rewarded with a minikit.