What perk? Because if you engage FPs as Firebug, Commando or SWAT, thats on you. Dont. Just dont.
For other perks FP is fine. Takes some practise to kill them fast, but lots of classes can.
Demo can knock them down with RPG to the legs and kill them before they get up. 3 shots total.
Gunslinger kills FP in 8 Magnum headshots. Takes less then 2 seconds.
Support has M4. Headshots, again.
Sharp can freeze, then kill them savely.
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General rules:
Every perk except Demo should headshot. Trash killers (SWAT, Commando, Firebug) should ignore FP.
If its raged and hits a player, it'll loose the rage. Dont just spam at it, you'll just rage it over and over. Let come down, get a dedicated Perk in position to kill it.
Or allow Sharp to freeze or SWAT to stun it, then focus fire headshots.
If you, as a team and as a player just spam bodyshots and rage it over and over again, its not the games fault. Play smarter.