From my experience, Covie weapons take down shields super fast(except the Carbine, it takes shields down a bit quicker than the DMR though), the UNSC weapons are best for straight ballistic damage, and the Promethean weapons are a mix of both.
Light Rifle: Like DMR when zoomed like Br Out zoom
Boltshot:Very weak pistol but powerful close up shotgun
Pulse gernades: they kinda suck but are good for closing players or enemies in a no access zone
Supressor: It like Ar but deadlier upclose
Scattershot: Its like the shotgun but with faster shooting cause its a bit weaker
Personally Promethan weapons are good at both heatlh and shields…but humans are better at heatlh while covenant are better at shields…Knights have shields and the are equivalent to elites shield in reach on legendary… plasma pisto works great on them…
To kill knights best weapons are:
Plasma Pistol
Any battle rifle
Shotgun in some cases
Any heavy weapon…rockets, sniper, e.t.c
your best advice however is to use everything you got to kill a knight give them no breathing space or there shields will recharge fast as hell