I want to show a few ways people can use the engineer class to their best.
When using the engineer class offensively, i reccommend using the SCAR-L or UMP, as both have good sights, high power, and good accuracy, but aren't the best in rate of fire. The M1A1 Thompson is a good weapon, too. For the sidearm, i recommend using the Tracer dart, allowing you to lock on to vehicles, if you can get them tracer'd. For the Gadget, i recommend the RPG-7 if the map is in abundance of heavy vehicles (More tanks), or the M2 CG if the map is in abundance with lighter vehicles (Helicopters, Humvees, Boats). The Spec 1 being used should be the explosive upgrade, allowing you to destroy many a tank before you are destroyed. The spec 2 should be improved demolitions, which will help destroy even more tanks, by giving your rockets a boost in power. Remember to tracer dart tanks and helicopters, stay hidden, only opening fire on enemy soldiers at close range, or enemy vehicles you have a clear shot at. Repair friendly vehicles. Also destroy buildings with enemies endangering your teammates. This is as easy as spotting people in a building, and blowing a section of a wall to oblivion right when they take cover behind it. And also, SPOT, SPOT, SPOT!
If going on the defense while using the engineer class, the best weapon will probably be the UZI or AKS-74u Krinkov. Both have high rates of fire. The Uzi has good accuracy but low power, while the AK is on the flip side- high power but low accuracy. The M14 EBR can also be used, as it is more useful at long range than the other SMGs and has good power. No matter what weapon you choose, your gadget should be either the same as above (M2 for light vehile maps, RPG for heavy vehicle maps), or the AT mine. Either way, using the explosives pouch will let you hold out longer with your rockets. The spec 2 for this strategy will vary. You may want improved demolitions for extra explosive damage, Body armor, for extra health, or marksman training, if using the AK. Remember, repair friendly vehicles, mine common routes if using the AT mine, engage enemies at close range, fire at enemy vehicles if you have a good shot if you have the RPG or M2 CG. And also, SPOT, SPOT, SPOT!
This is a more specific offensive class, as it requires stealth (Believe it or not, this actually defies Zealot's law of bad company 2). The primary can be any SMG, though i reccommend the XM8 C. Any SMG does not include any weapon without a suppressor. Though you won't show up on the minimap even if you are firing an unsuppressed weapon, the silencers reduce muzzle flash and noise, helping you stay stealthy. You will want to use a tracer dart, which you can use to mark enemy helicpoters as soon as they take off, allowing the chopper to be shot down by other engineers, while giving you assist points and keeping your position secret. With this class, none of the launchers are useful. AT mines may be fairly useful, as you can plant them infront of a tank, then when the enemy inevitably drives forward when they get in, they go BOOM. The spec 1 would be extra explosives (again), and the spec 2 should be marksman training SMG, as this will increase your accuracy, meaning more shots hitting the target, and faster kills, and less bullets wasted, and less time firing, meaning less time for the enemy to find you. Remember, if playing rush, try to secure an MCOM, then arm it and camp it. If playing conquest, don't try to capture it, as this will alert enemies to your position. Instead, tell your squadmates to come secure the flag, and that you have kept them suppressed. Also, mine enemy vehicles, Once friendlies secure the area, repair friendly tanks that were damaged in the fight. Remember to, SPOT, SPOT, SPOT, as this will let your team know where the enemy positions are. Also, try to be on extreme alert, because if an enemy SPOT, SPOT, SPOTs (Isn't it fun to say SPOT, SPOT, SPOT like that?) you, you're dead.
Note: This will be updated if i think of any more.