how to get a TF2 Parasite Hat (Hat Trick achievement)

first things first - you have to enable developer console to be able to add bots in the game

open OPTIONS, then KEYBOARD/MOUSE
toggle DEVELOPER CONSOLE on

select PLAY ONLINE, then CREATE GAME

you can select any DIFFICULTY you want. EASY is extremely easy, honestly, if you want some challenge then start on at least NORMAL
enable FRIENDS-ONLY GAME

click on that icon in the corner
and select WOLFE character (he is of SPECIAL class, has bonuses to health and damage, good option for novice)
Wolfe's equipment should be:

• S23A SynTek Autogun (has auto-aim option)
  
• IAF Advanced Sentry Gun (helps at the end of mission)
  
• IAF Personal Healing Kit (in case if you will be damaged heavily and would be no time to use IAF Heal Beacon)

open the console (~ button, below ESC button) and type the following commands (one after another)
that will add three bots into your game:

• Jaeger (an OFFICER class, very powerful with Vindicator weapon)
  
• Faith (a MEDIC class, not-good soldier, great healing capabilities)
  
• Vegas (a TECH class, has more health and will help with unweilding doors and installing turrets)

reconfigure their equipment in the following way:
Jaeger

• M42 Vindicator
  
• M42 Vindicator (with two Vindicators he will need no ammo for a long time and will be highly effective)
  
• Hand Welder (to unweild one door)

Faith

• 22A3-1 Assault Rifle (just a good gun)
  
• IAF Heal Beacon (to heal everyone in intermissions)
  
• IAF Personal Healing Kit (extra healing if things go wrong)

Vegas

• 22A7-Z Prototype Assault Rifle (has auto-aim option)
  
• IAF Ammo Satchel (in case of anyone will be out of ammo)
  
• IAF Personal Healing Kit (extra healing if things go wrong)

when you will be ready press START MISSION

• press 1 to use a medikit
  
• press X to give marines a "hold position" order
  
• press V to give marines a "follow me" order
  
• press F1-F2-F3-F4 to switch between marines (maybe useful if you want to use someone's medikit, but you better stick with Wolfe for the whole level)
  
• if you have switched marine, then use a "follow me" order to prevent bots from being stuck

follow the level until you see boxes. destroy them with your weapon or with melee attack (RMB)

you see a computer, press E to make Vegas, your tech, to hack it

from that point swarm start attack you. fight them all and move forward (no matter what way you take on intersection, they all lead the same direction)

remember, Wolfe's S23A SynTek Autogun has auto-aim function

at some point you find a wielded door. press 2 to give Jaeger a command to use his Hand Welder to open the door for you

move forward killing the swarm until get to the outdoor

in next part Vegas, your tech, must hack a computer while the swarm will attack from all direction. you better to prepare yourself for the upcoming siege, place few Sentry Gun, heal and recover ammo

game will hint you to place a Sentry Gun at that position

but you instead place it here (to cover wider range and give Vegas, your Tech, extra protection). to place a Sentry Gun select it with MouseWheel (or Q) and press E

there is a second Sentry Gun on the level; you can retrieve it and assemble for extra weapon power. it is located here (check the map in the corner), press 2 to ask Jaeger to unweild door for you

place Sentry Gun whenever you want, but better to face to the left or down like here

press 3 to make Faith, your medic, to drop a Heal Beacon and heal your all

press 4 to make Vegas, your tech, to drop a Ammo Satchel and recover your ammunition

whenever you ready, press E near computer to ask your Tech to hack it

protect him meanwhile (with two turrets it will be a cake, even on Hard difficulty)

mind you that swarm will come from the right door (they will destroy the door), so be prepared to face them

whenever Vegas finished his job, move the whole squad right and finish the mission

in order to get a Hat Trick achievement (and a Parasite Hat) you need to play two missions, but you actually can just restart that mission and complete it second time - it will count too

on that screen press ESC button

select CALL A VOTE

select RESTART MISSION in the corner

that's all. after finishing that mission second time (try on harder difficulty, tho!) you will get a Hat Trick achievement and a Parasite Hat for Team Fortress 2

( image from https://wiki.teamfortress.com/ )

if you enjoyed Alien Swarm and would like to play it further, then consider checking semi-official mod for that game - Alien Swarm: Reactive Drop
https://store.steampowered.com/app/563560/Alien_Swarm_Reactive_Drop/

that mode has lots of additional features and fixes that make that game way better. in includes:

• more campaigns and maps
  
• more weapons and items (gas grenades, desert eagle, devastor, etc)
  
• more challenges to make game harder or different (like mutators in Left 4 Dead 2)
  
• fixes for bots' AI and full-term singleplayer campaign
  
• 8-players support
  
• deathmatch and other PvP modes
  
• a Steam Workshop support
  
• leaderboards, speedruns, stats, achievements

if you want to thank me...

...then check my guide about fixing various game issues: launching, crashing, optimization, stuttering, gamepads, etc. the guide is huge, but the topic is broad to begin with
https://steamcommunity.com/sharedfiles/filedetails/?id=1935376439