Move
The best tactic for being successful in Dan the Man is to keep on the move so you don't get surrounded by heavily armed enemies and jump after you get off a few shots or combos. When you do this you will be giving the enemies little time to respond and you will find many of the enemies will be unable to deal with you when you are in the air.
Distance
Make the enemies that are armed with firearms who try to shoot you from a distance your priority and deal with them using your throwing or shooting weapons.
In-Game Shop
Although the items you can buy in the in-game shop will help you in tough battles it is more beneficial to save your Gold for the character move Upgrades.
Checkpoints
The levels in Dan the Man are quite long so make sure when a checkpoint appears you take the opportunity to save your game otherwise you could find yourself starting all the way back at the beginning of the level if you die.
Hidden Areas
Take advantage of your Characters ability to backtrack and jump up or down when the situation presents itself as it can help you find hidden areas.
Water
Remember that although falling into the water will not instantly kill you or the enemy it will cause damage. Use this to your advantage by knocking the enemy into the water.