The game's good as long as you have someone to compete with.
Or if you are really in for getting those achievements.
The game starts off in a very interesting fashion, with you having to kill a few people and then some. It isn't too hard and after a couple of missions you pretty much get used to it.
Here your main aim is to kill the boss. Nobody else matters. If you do kill a few others, well, they just add to your experience, so no harm done.
KILL THEM ALL / AMBUSH
You'll have to kill every person in the mission for completion. Ambushes work in the same way, except when it's an ambush, everyone knows you are already on the scene and they take places and start shooting you from the word "Go"
Here, the main thing is to tranquilize the boss. Be careful though, if you kill him / her accidentally, the mission fails. The mission also fails if you tranquilize an enemy, because they have to be killed not just put to sleep.
The weapons are easy to understand, and of course the best ones cost the most. Your arsenal consists ot 5 types of weapons: handguns, snipers, automatic, shotguns and exotic. You'll have to buy the best of them to play the weapon challenges. They are tricky but pay well.
ARMOR / MEDS
These will come in handy after you've reached level 10 or so. Personally, I haven't used armor since it looks like a waste of money compared to the meds. The meds aren't very cheap either, so you need to be very careful and start saving up for stocking them.
1. Do not buy your weapons hastily. Save up a bit and then try to buy a better one, preferably with a higher range. It's much easier to complete missions with a much better weapon than a slightly better one. As an example, my second sniper rifle (in addition to the one that was given to me in the beginning) was a Cheetah Rifle costing 250,000.
2. If you have the credits, save them. Two informants later will be a good place to spend them. Shady Wessler is the best buy at 5 credits and then you have Parker who comes for another 5 credits.
3. Headshots give you twice the XP. This will help you level up faster. The guys with masks on cannot be killed by headshots; hit them in the mid-section instead.
4. Always try and locate the boss and kill / tranquilize him / her first. Unless your mission mentions that you have to kill his goons befor the boss arrives. If you don't kill the boss first, the boss usually escapes and you fail the mission.
5. There are a few bugs which cause the boss to escape before you even see him. Don't worry, just keep restarting the mission. You lose energy, but after 3 - 4 retries, it becomes an ambush. Now, no one runs and you get to kill everyone for the XP.