To easily kill the Nazis armed with the particle cannons use your Mire power and aim for the red targets on their shoulders. A new target will then pop up from their back when both shoulder targets have been destroyed and they will then die easily when this target is then shot.
To be able to rank up you must collect Gold. Gold is awarded for killing enemies, dealing damage, healing allies, dispensing ammo or health packs, completing objectives, and so on. Collected Gold can be used to purchase new abilities, as well as upgrades for available weapons.
Save $17,000 by not upgrading Veil powers (Veil, Shield, Empower or Mire) because when you have collected the final 'Tome of Power' in the game you will automatically be rewarded with ALL of the skills for each power available for purchase. The money you saved can then be used to upgrade your weapons.
When you have Completed the game the cheat menu with 'Pumpkin Heads', 'Infinite Money', 'Unlock All Veil Powers', and 'Unlock All Weapons' options will become available. Enabling these cheats will prevent trophies from being earned.