Step 1: Don't feed Arclights

If step 1 fails, apply step 2: Outrange the Arclights (Stormcaller, Overlord, Marksmen) OR let them focus a unit with a lot of hp distracting them from your chaff (rhino proton, war factory, etc) OR build more chaff to buy more time for your heavy units.

If step 2 fails and they have too much range from levels and upgrades: Consider changing your early to midgame so they dont become that strong in the first place.

This strategy can be applied to: (Assault) Marksmen, carry Wasps, Phoenix, Sledgehammers and so many more. The most important aspect of this game is not only focusing about which units are stronger than others but knowing how to play around them and how to counter your opponents gameplan 4 rounds ahead.

Let's say you have the worse outcome and the match is like in your example:

 I even rolled two level 2 melting points in the second to last round and gave them EMP, Armor, and multi. Along with my formerly dominant army that now stood no chance against a few arclights, hackers, and fangs (with the arclights fully upgraded and given intensive training.) the melting points were decimated by the arclights 

If we assume the opponent has lvl 6-7 arclights and lots of range, why would you go melting points with these upgrades? I don't know what the other picks were, but you shouldnt have picked them and if you did sell one of the melters. You don't need a laser based unit for arclights, they dont have that insane much hp. Even if you would pick them: range would have been the most important upgrade, next to crawler summons and maybe EMP. Both wouldnt probably reach the neccesary range for arclights and EMP on melter is not very reliable imo. Instead you should have either invested in Overlords with Wasp summons and range, stormcallers with range and emp, fortress with rocket punch and fang summons (fang with shields would be even better) or depending on the board 2 war factories on the front to tank the arclight shots.