Normal isnt too bad. Learn the boss patterns and exploit their weaknesses and you will have a better chance of downing em quicker. Also another good tip is if you are struggling, fight in the open world to achieve 20 kills as Lan stated in his guide. When it starts to rain, a mini boss will appear. It could be a mutant, a Droid squad, etc. After beating it a thug with a box will appear. Kill him and take his power up. You can carry up to two power up items into a mission. Hope this helps.