Once you purchase the armor upgrades from the vendors the game becomes way too easy. I personally thought the end of the game would be the hardest (because of all the enemies) but I completely forgot that the Resistance gives you the extremely powerful Authority Pulse Cannon. Plus, ammunition is very plentiful throughout the last mission.
135/136 games completed (including DLC)
Completion percentage: 99.40%
Latest retail completion: Earth Defense Force IA
Latest arcade completion: Warface