I think the idea for a Sniper type beam would be best.
To make it less geared twords hitting smaller ships. It should NOT be aimable. and only shoot forward.
As for the graphics, is there a way to make a "beam weapon" weapon that hits nearly instantly but has a special effect trail that lingers for a few seconds?
Like if you used the trail on cruiser forward guns or armoured transport engines. not sure if those are interchangable.