Just Cause 2: Multiplayer Mod

Just go on any server you like and well be AFK, You can AFK on =[DBG]= Dbeast Gaming server, which has a invincibility command "/passive" which makes you immortal and you can afk on Mile High Club (Hold G button, press MHC)

And you just wait for those achievements to pop up.

You can cap out the max fps for the game using prompts of steam launch option (Click RMB on JC2:MP → Properties → Launch Options) and type /frameratecap = 1
and enter the server so the usage of your CPU and GPU will be low.

As achievement suggests just spawn a Vehicle on =[DBG]= Dbeast Gaming Server using B button, select planes and choose any you like, but I suggest you use "Si-47 Leopard", do this while you fly in air (Free fly) and when you spawn a plane just aim up and press TAB to get highest speed, and achievement should be yours.

All of those achievement don't require lots of people to kill, you just need one person to help you (Or maybe a mod/admin to TP people to Oil Rig)

Edit: It seems that the best server for kills is =[DBG]= Dbeast Gaming, you can use a command on each location to spawn there everytime without the need to fall from sky (/setspawn) after it's set you can AFK and the player you're helping/ he's helping can get kills.

For stuntshooter just ask a person who's willing to help with kills stand on stil plane and kill them while they are standing on it, you don't need to go up in air or go forward with it.

For Power Surge you need to be on that location (X:25460; Y:17870) "Awan Sendawan Power Plant" and the person willing to help just TP's to you using H menu (Works only on Alpha's server) and presses "Go There!", or you can Whitelist him so you don't need to click "Accept" everytime (Cooldown after kill to TP is 15s)

For Oil for Blood achievement just TP to any Oil Rig using F1 map to pinpoint any oil rig, then press M and teleport to yellow square you'll see on map (For example: X: 31421 Y:7270) While you stand on it you can ask a friend to TP and kill him 100 times.

As for Firestarter you can use any location to kill and get 1k kills, usually people tend to help you with them if you ask in global chat on Alpha's server, so even they can help.