When Large Zeds are approaching, everyone should do their job. If you want XP to level up, you can just shoot with your 9mm Pistol once and you'll get the full XP once it's killed.
In fact, you shouldn't even shoot them because Large Zeds rage after taking a small amount of damage, and enraging multiple of them at the same time may cause a team wipe. If you're not playing a Large Zed killer, it's not your job to kill them. (Don't forget to read the description on how to use the map) If you want to practice solo kills, you can download the map HERE It'll be much harder to kill Large Zeds in an actual game. Remember, these videos were recorded on a test map, where I'm not under any pressure from the horde surrounding and teammates randomly shooting. Swat, Commando, Medic and Firebug are not supposed to kill them but I still listed them in case you need to do something in an emergency. They all have their advantages & disadvantages. Primary Large Zed killers are Demolitionist and Sharpshooter, while the secondaries are Gunslinger, Support and Berserker. Mandatory skills and weapons are outlined with yellow color. I listed the loadout and skills needed to perform each kill. Level 25 is required, but you can still perform some of these methods to a degree even if your level is lower, especially on lower difficulties. Everything is based on 6 players Hell on Earth difficulty, so they can be applied on lower difficulties too, just easier.