You're quite correct, Scaramouche, the heat ray would be entirely enough. It has an unlimited range, will destroy anything solid that it hits (including solid steel), and you can fire it on "open sights", because it follows a straight line. Ergo...you can't miss once you're sighted in.

The weapon shown in the movie works essentially the same way, although it may not look exactly like what you saw in the old Classics comic book (which was pretty cool).

The only way to hide from the Martian fire would be to be underground or hidden behind the crest of a hill...or just to be hidden well enough that they don't know you're there. The latter technique was used briefly by the Royal Artillery Corps in Wells' book. They hid their guns in groves of trees and other natural cover, and waited for the Martian machines to approach, then got in a quick volley or two. By this means they destroyed a Martian machine near Maidenhead, as I recall, hitting it full in the hood. They were then quickly wiped out by several other machines. After that incident the Martians began firing cylinders of a poisonous black gas into every clump of trees or other cover, ahead of them as they went. That nullified the British artillery, killing the crews or driving them off in a panicked rout. The only thing that could be done after that was to leave triggered explosive booby traps for the Martians to stumble over...a passive and not very effective defence, but better than nothing.

The reason the "Thunder Child" managed to destroy a Martian machine with its guns was mostly because they were puzzled by the approaching ship, having never seen one in action before. That gave the ironclad one chance, and they made the best of it. A second Martian machine went down when the exploding and sinking ship collided with it under full steam ahead and dragged it under the waves...

Again, those scenes were in the book, not this new movie.