i think your best bet is on tc bows then. Green Slime Limbs, Thaumium Plate, and Fiery String with all possible modifications spent on Haste for the longbow. Manyullyn Heads, Bone, and Blue Slime fletching maxed out on nether quartz for the arrows. if youre looking for a gun i dont really know flan's so i cant help with that
How much damage do they deal (1 point of damage = 0.5 heart) at maximum? Can those bows deal explosive damage (Significantly stronger against some types of armor) or do they hitscan or neither? Railgun from MPS deals 15 hearts/30 damage (hitscan) and Plasma cannon deals 20 hearts/40 damage (explosive) when fully charged.
hmm, lets get creative. I like to start off with a Sticky Grenade, then as they try to run from the nade(thats stuck on them), take out your p90 and push them back a bit. Then take out your .50 cal and finish them off.
also with metalurgy you can also include some extra effects depending on its intended use. for example 1 metal if included will give weakness 2, another slowness2, poison firerly wither ect...