Defiance is the first MMO that combines the strategy of an MMO with a shooter so if you want to make progress you will need to have decent shooting skills and understand how to mod the weapons, level them up and also get more EGO points and powers to compliment your playstyle.
If you are just expecting a basic shooter then you will need to think again, as the strategy aspect of Defiance is quite deep. You get EGO powers that will affect your character in different ways and you will need to use these in order to compliment your playstyle. There are thousands of different combinations that you can have using the EGO grid and if you are not sure what you are doing you may end up underpowered in the game.
If you need help and want to make sure you progress through the game I suggest that you check out Defiance Guru. It is a guide that has been put together by a pro gamer and features a step by step missions guide, a full guide to the best perks and EGO powers and you will be able to speed through the game.
There is also a section on general shooting skills and how to get your game up on this aspect of the game to dominate.
If you want the EGE check out Defiance Guru here >> Defiance Guru - Ultimate Mmo Guide