Just Cause 2: Multiplayer Mod

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.

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

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.