Pro tip: if you want to get the 17 minimum achievements to unlock all the weapons, just set up a local server with bots. As long as you don't enable cheats you can get achievements. Set the bots as easy and they are pretty dumb.
It's very easy to get the following achievements this way. I suggest using dustbowl as the map. I made mine 32 players because sometimes you'll want to add more bots, and sometimes just 1 is enough.
Land Grab
Help a team member construct a building. [probably the easiest. just have another engy on your team]
Battle Rustler
Teleport 100 team members into battle. [must teleport a certain minimum distance. set the enemy team as heavy weapons, and have your team be scouts. they will take your tele and die quick, respawn, and take it again and again. just keep a decent ratio or your scouts will overpower the heavies and win]
Built to Last
Help a single building take over 2000 damage without being destroyed. [enemy soldiers work better than demos. demos always killed my SG too easily]
Doc Holiday
Have a dispenser heal three teammates at the same time.
Frontier Justice
Within 10 seconds, have your sentry kill the enemy that just killed you.
Silent Pardner
Upgrade 50 buildings built by other players. [just set up a team full of engineers and you can get this one fast. once they all build and everything is upgraded, just kick all bots and add more engineers]
Revengineering
Use a revenge crit to kill the player that destroyed your sentry gun
Deputized
Get 10 assists with another Engineer where a sentry gun was involved in the kill. [set up a team with you and another engy, and the other team is just scouts. won't take much time at all]
Doc, Stock and Barrel
Repair a sentry gun under fire while being healed by a Medic [you will need at least one other non-medic class on your team to get the medic to leave your spawn. i found if it's just you and a medic, the medic will never leave. not sure why]
No Man's Land
Use a sentry gun to kill 25 enemy players trying to capture a point. [set up a level 2 sentry near a point and set the other team as scouts. if you upgrade to level 3 it will kill the scouts before they can get on the point. the first point in the first stage of dustbowl works the best. just set it in the corner by the stairs.]
If You Build It, They Will Die
Haul a level 3 sentry gun into a position that achieves a kill shortly after being redeployed. [bots focus more on killing enemies than capping a point, so move your SG up while your bots are fighting. chances are you can set one down in the middle of a fight]
The Extinguished Gentleman
Have dispensers you built extinguish 20 burning players.
Texas Two-Step
Use your shotgun to finish off an enemy recently damaged by your sentry gun.
Uncivil Engineer
Provide an enemy player with a freeze cam of you and the sentry that just killed them. [I was pleasantly surprised to see this work with bots]
Unforgiven
Kill three enemies with revenge crits without dying.
Pownd on the Range
Kill 10 enemies outside the normal sentry gun range using the Wrangler.
Building Block
Have a sentry shielded by the Wrangler absorb 500 damage without being destroyed.
Breaking Morant
Kill 10 Snipers with a sentry gun under the control of your Wrangler
It's also probably the quickest way to get 5000 kills, just set the enemy team as all scouts and make sure they can't hit your sentry from a distance [several scouts shooting their pistol will kill your SG], and make sure you're set up so they can't circle around you. One time they were able to completely surround me and kill me and my buildings.
You cannot get "Trade Secrets - Kill 20 players carrying the intelligence" because CTF maps do not work with bots. Also I read spies don't really work so don't try for those related achievements.