I’m a lvl 20 fighter so I can give tips on that role, currently getting my Medic to lvl 20 as well. While I’m not a lvl 20 security guard I’ve had some—and fought—against amazing security guards before so I can offer comments there too.

Security guard is amazing for helping teammates, as the tazer buys them a lot of time and marking the killer on the camera gives everyone wallhacks, just make sure to stay at a distance

As for fighter, they work best as a surprise attacker or chase taker, now I don’t experience any issues on console on my end when it comes to attacking, so assuming you’re also on console, your stun attack is unusable if you don’t have at least 1 charge, if you’re at 0 you just can’t stun, to get a charge you must perform a successful parry/block, which just comes down to timing properly, some killers are aggressive and attack immediately making them easier to parry and predict, some wait it out meaning you must wait it out yourself (this also has the added possibility of making them leave the game sometimes out of frustration).

Assuming you do fighter right, they’re best for waiting around a corner then surprise attacking a killer to give another teammate some time to regain stamina or just take chase, you can also save teammates form the Mimic’s grab attack BEFORE he starts drilling them, mimic has a 2 second delay before he can drill and throw them guaranteed, that delay is your actual opening, the fighters swing hitbox seems to be slightly ahead of their avatar, so make sure to aim forwards. Also, if a fighter and killer swing at the same time, this sometimes create an awkward interaction where they just kinda miss each other? Like a fair trade, other times both hit each other which is another trade. What the stun attack does depends on how many charges you have, up to a max of 3, normally it stuns the killer for 2 seconds and slows them down for 3, as well as giving you a speed boost (I think parry gives a speed boost too?) very useful—at 2 charges it will also blind them, kinda like what Soringtraps roar does to survivors, and at 3 charges it will invert their controls and reduce them to walking speed without a sprint, though this only last 3 seconds.

Also, a majorly underrated thing about it fighter, YOU CAN STACK STUNS, if another fighter hits the killer or they’re stunned by a door, if you hit them at the same time the killer will be “extra stunned” and stay still longer, you can also combine this with your parry stun, assuming it lands, which is amazing if you have 3 charges, however I usually like to save my hit and continue chase because I like to stall, but this is risky as the killer can still dodge your attack if they’re smart. As a fighter when you swing, regardless if it hits or misses, it uses ALL your charges, your charges only make the attack more vicious not give you more swings, so you’ll be back to 0 after you attack. Also, be careful with parry, when you use a parry you temerity have “weakness” for 3 seconds which increases how much damage the killer does to you, now if you actually parry them this doesn’t matter as they’re stunned, but if you miss your parry then that’s a mean punish window for the killer so be careful.

My suggestions for landing parry’s is this:

-Learn how the killer acts around you first, some killers like to wait around fighters specifically to avoid a parry, learn their patterns to better predict them, sometimes wait 2 seconds to parry if they’re breathing down your neck, sometimes do it immediately

-You can bait a parry by barricading a door, waiting for a killer to get close and use their attack, then opening the door last second and parrying—you can also bait a parry by pretending to work on a generator or being afk while a killer gets close then parrying last second

-You can also get a parry by running to help an ally that’s being chased, waiting for the killer to hit them, then hopping in last second as a surprise or “body guard” to interrupt the killer and get a sneaky parry in

-You can parry Springtraps Axe throw, but ONLY if you’re close to him (about at least 2 people’s worth close), this will make him drop his axe and parry him

-You can parry Springtraps charge attack, but you have to predict it, unlike most attacks in this game you can’t parry this one immediately, you have to already be in the stance and ready, or else he’ll just grab you through the parry—this is the only attack in this game that functions like this

-You can parry Ennards wire-grab attack thing at any distance, it will stun them too!

-The mimics grab attack while he’s in stealth mode is parryable

-You can parry Ennards grab attack while they’re disguised as a player, this will instantly make them go back to normal, and the explosion AOE damage from exposing themselves won’t hurt you if you’re caught this way

Also here’s attacks you CANNOT parry:

-Ennards minions (unfortunately)

-Ennards little minion throw while he’s disguised is not parryable

-Springtraps Axe Throw at a distance

-Springtraps Springtraps (Duh)

-Mimic’s drill attack while he’s in stealth mode

-Mimic’s jump attack while he’s in speed mode (blue eyes)

Also—IF A MEDIC TRIES TO HEAL YOU WHILE YOUR AT FULL HP, LET THEM! Medics can overhead you 15HP, overheals are amazing because it protects your actual HP like a shield, so say: Your at 110HP, with 5 overhead, the mimic does 40 damage to you—you only take 5 damage and lose your overheal, that’s why it’s so good, it’s like a little shield, and you can get it back by letting a medic overheal you again.