Remember to access the missions through the campaign selection. I played in vs, mode. The bots seemed a little easier there. Throughout my playtime, I only encoutered one other player, so don't expect much help.
I recommend starting a new character and holding him at level one. Reset completely after each level, even unsucessful ones. Complete the challenges to at least tier two to unlock all guns and mods. They give you the only advantage you'll have.
You should always be the objective's class, as your teammates are often useless. When you have an universal goal, I recommend Engineer. The damage boost is the most important buff. The soldier's fire grenade is nice, and you won't ever run out of ammo. It's never worthwhile to give a teammate ammo. The medic's self heal takes too long to be of much use. The operative is just bad. My favorite weapons were:
Mossington Shotgun with Speed sling. With a damage boost, this will one-hit kill at two meters. Otherwise, one shot will stop short of a one-hit kill. Camp next to a defend objective and shoot anyone coming for it. Useful on offense, but not overwhelming.
Euston AR with High-capacity mag, red dot sight (I like Yeotek), and speed sling or Front grip. All-purpose weapon. If you want more range, try the Rhet AR.
Lobster GL. This weapon is too random for most levels, but if you're having trouble with a well-defended objective, you could give it a try. It's good at disorganizing opponents.
Tampa SMG with CoGA scope, High capacity magazine and Speed sling or front grip. Go with the speed sling if you have the shotgun in front, otherwise they're even; use whichever you like best. If you use the lobster, you may want to switch to a red dot sight and Muzzle brake. Short, controlled bursts are a must.
Barnett light rifle with COGA Scope and Speed Sling. Most of brink's levels don't reward sniping, but this is the best long range option that is still useful as a backup.
Day 1: Hostage Rescue. Bring a Shotgun and SMG. The objective room will always have bots hiding. Make sure you have two bars of energy before entering. Rapidly throw both grenades at enemies, then use your shotgun to finish them. Quickly attach the bomb, then guard it. The second part is really easy. Make sure to capture the command posts.
Day 3: Smash and Grab. The mainframe area is perfect for the shotgun. It's an easy hack. Change to engineer and an AR/SMG combo. Cracking the safe is tough, but they can't undo your progress so you will eventually succeed. Then there's the run out. This is tough. Exit the safe room on the side of your spawn, run up the escalator, then go straight and turn left when you must.
Day 4: Dirty Bomb is easy enough. AR/SMG or AR/LR is the best combo here. Most of the objectives are generous with the time given. Delivering the sample is hard, but you should have 15 mins to try and get lucky.
Day 5: Prison Break. Play engineer throughout and use your mines. Use whatever weapons you like best. The size of this map makes it easy to get lost or be in the wrong place.Once you know where to go, Defending the conduit and Stopping the prisoner are both fairly easy.Day 6: Early launch.
I think this is the hardest mission in the game. If you can, set up a boosting session to get this. It took me over 50 tries to succeed against bots. This is the strategy I used. If you keep trying I'm sure you'll get it.
For the first part, Shotgun/SMG is best. Rush the crane controls before the barriers are up. While you're repairing it, couch and use the panel for cover. You can hold out surprisingly long from this position.
For Hacking the missile controls, You'll want to capture the health command post (
will give directions). Take the stairs from there and look at the hack area. You want to plan how you're going to take these guys out. I recommend grenading one and jumping at another with a damage-boosted shotgun. If there's a third, you should have enough health to take him down, but a fourth guy in that area is deadly. Quickly place the hack box, then retreat to underneath the area you jumped from. There's a small, easily defended area. You should have time to shotgun anyone entering. If the hack percentage goes down, quickly exit, kill anyone standing next to the hackbox and rehack.
Day 8: Fallout. This one is very easy. Use whatever weapons you like.
Day 1: Getting answers. Defending the door is hard, but hold out as long as you can. Stopping the agent is easy. Capture the health station, construct the stairs MG, then go try to slow down the agent. If they reach the stairs, Grab the MG. Keeping the agent down is easy with it.
Day 2: Break out. This one isn't hard, but you do need to rely on your ally bots. There's a lot of random factors that go into winning this one. When hacking the safe, use the glass to shield yourself from enemies, then shotgun them once they close. Change to engineer before delivering the code, so you can one-hit kill with the shotgun. Be sure to buff your allies while escorting Nechayev.
Day 3: Chen's Plans. Again, the mainframe room is a great place for the shotgun. The same factors that made this mission tough for security make it easy for you.
Day 6: Black Box. Approach the pillar just like the first door. Both grenades, then kill before they get up. Guarding the bomb is tougher here, but not impossible.The bridge is best rushed. The enemy has a lot of trouble removing it, so if you stick it, it's good. You can probably ignore escorting the bot and race right to the hackbox, slipping by the defenses. Once you can, started hacking and hide behind the nearby table, next to a back entrance. From here, you should be able to surprise any defenders rushing in.
Day 7: Attack on CCity. The generous time limit hurts you here. I lost in overtime dozens of times. Try it with a higher-level heavy with a Chinzor MG (high-capacity magazine). Right next the bomb site is a hole that you can shoot the soldiers from. If the enemy reaches the crane control, I usually reloaded, the second half of this mission is hard.
Day 8: airborne. This is also easy. You have plenty of time and each of the objectives has cover. When delivering things to the plane, use the back entrance. The enemy won't guard it at all.
EDIT: I've sold this game and am no longer able to help people. Sorry.