Having an issue with Arachnids Biter lvl4, it seems when they initially die and leave an egg the only thing i have found to kill them quickly is the defender drone. I so far have tried flamethrower, works every now and then but have to jiggle the mouse vigorously to make it hit, rocket launcher does nothing, bullets from rifle and turret does nothing, flame thrower turret does nothing, im wondering if im missing something.

You may have too few ammo turrets or too many arachnids attacking you. The egg must be destroyed within 3-5 seconds or else it will spawn a new arachnid.
This faction is vulnerable to physical damage, but is difficult to kill with other types of damage. Ammo turrets are great for destroying arachnids. Of course, it is necessary to research damage and rate of fire.

At one point had about 15ish T1 Turrets attacting it with T2 ammo, they were not even damaging it. I could kill them in like 1 second with the defenders as they are drones. As for the rate of fire i had it researched as much as possible, has this happened before to anyone else?

Apparently, one of your mods removed the homing ability of the ammo. If your ammo is a projectile and someone forgot to make it collide with the drones, then they will fly right through without damaging them.
I know Kratorio 2 has something similar, but the turrets work just fine there.
In your case, I can't help. The mod that changed the bullets should fix the collision layers for its bullets. Before that, you better disable the Arachnids faction.