They would strip off the leg armor in one shot so you could then shoot the exposed leg with your primary weapon. The devs called it the "leg meta" and agreed that it didn't really make sense for that to be a better strategy than just shooting it in the head.
It's 3 shots with the auto cannon in the tail and they bleed out. Same thing can be done with a lot of the primary weapons, it just takes a good amount of shots. Even the sickle can do it after changing heat sinks a couple times, you just have to matador them.
And by other bugs you mean 4 other chargers and 3 bile titans. Playing on higher difficulties, effective anti tank is a must. We used to run 2x EAT, now we run 3x quasar since it has unlimited ammo and isn't a pain to drop it all the time
The butt is an explosive weak point. Non explosive weapons do 10% damage when striking it, where explosive type weapons do full damage. Charger butt, bile variant side sacks and the titan belly are all examples of these weak point and why most primary weapons struggle to kill these enemies.
Not quite, railgun had superior rate of fire and pierce, so it could clear entire waves if properly kited. Quasar does a lot of damage but it can't even oneshot a berserker.