This is a rather complicated question since I don't know how to redact the title. But I'll get into specifics.
In my world they're is two types of humans Conventional and Contemporary (Temporary names), conventional humans are just that, regular humans with access to conventional technology while contemporaries are humans that look regular but have access to magic of sorts. Conventional humans cannot replicate magic, they physically cannot (and since magic does not abide to all laws of physics conventionals have a hard time understanding it) while contemporaries don't use regular technology for cultural reasons.
One of the most common weapons amongst the magic bearing kind is a sort of beam weapon, it fires a plasma-like substance that follows a laser-like nature (Conventionals do not understand how this weapon it's even possible) and ranges from all sizes and forms, from a simple handheld weapon and that fires little bolts to massive artillery platforms firing More persistent beams.
The one kind of platform that regards to this question is the artillery platforms, they can fire beams of this substance at 5,000°~6,000° Celsius (5273.15K ~6273.15K) at ranges of 50 to 100 kilometers while the attack stays focused and is mostly unaffected by atmospheric conditions; naturally this is devastating to conventional armed forces as even the mightiest of battleships can be sunk on a single hit or entire fortresses leveled to the ground in a few barrages.
Naturally, this system also has weaknesses.
-Slow Beam: Despite following a laser-like behavior, the beam travels at a relative slow speed of Mach 4 (1,325.2 M/S) with hypersonic aircraft commonly outrunning the beam. (And since it's like a laser, the curvature of the planet can also act as a shield)
-Bad Targeting Systems: Magic can replicate many things that technology can with exceptions, computing power being one of them, as contemporary processing capability is laughably bad compared to its conventional counterpart, meaning that the weapon will have a hard time aiming at longer ranges or predicting the target's movements.(The most advanced magic processor can only achieve results similar an 1980's era computer)
-Overly big emitters: The more powerful the beam the larger the emitter, with the artillery platforms threatening battleships or fortresses being the size of big houses. (Contemporaries have even larger ones, yet they don't use them for the exact same reason)
Conventionals can't use magic and have to abide by conventional technology; the level of their science allows them to wide adopt fusion reactors, quantum computers for point defence systems alongside orbital assets like missile satelites.
¿Which weapon design could conventional humans use to counter this contemporary beam artillery?