Yes, it is tough to get used to. Well, in each level, there are plenty of hour glasses, so be sure to collect them in order to gain your time so you can kill more enemies, and by killing more enemies, it will raise your rank depending on how much enemies you kill. Plus, if you can, find a GROUP or a large amount of enemies bundled up together and take them all down in one shot using a grenade, therefore you will kill about 5 or more in a row, the more you kill in a row, the better you will do. So don't waste too much time in one spot, roam around to seek for some enemies.
If you finish the game on medium or hard you will find that you get infinate weapons to be able to unlock the rest of the mercs i would suggest to complete the other missions first ADA etc.
u only get certain weapons when you are doing the mercinaries, and when u unlock the handcannon you dont even get to use it in the mercs
That's because each and every character have allocated weapons.