Have all casters cast their spells at the same time, then avoid casting additional concentration spells
The big problem with hiding a spellcaster with a spell like Heat Metal is a combination of two things:
- Casting the spell is generally obvious (it has verbal and somatic components)
- The effect of the spell is immediate and obvious
If the spellcasters all ready spellcasting actions contingent on the last of them casting a spell, then all spells would be cast "at once" and it would be far from obvious which enemy cast which spell. Not casting additional concentration spells would prevent the target from deducing which of them must be maintaining Heat Metal.
Cast from one or more obscured locations
If their dreaded enemy is a highly capable fighter, it's unlikely these spellcasters would rush out to open ground and start fighting them. They can make use of terrain features like cover and concealment to make them harder to see, which could possibly obscure what they're doing. Sequential turn resolution destroys this, but that's not information that would be available to characters in the game.
Spellcasters can dart in and out of cover, use the Hide action, and similar to obscure what they're doing.
The shell game
There's a long-standing con in which players try to guess which shell (or cup, or whatever) is covering a pea (or something like that). The shells are moved around in an effort to distract the player such that they cannot track which shell covered the pea initially, making it hard to identify the correct shell. In real life, this is nearly always a scam (the pea is removed from the table).
But in your game the spellcasters could run out of sight, and if they all look the same as one another that would be enough to hide which one is maintaining the spell, whether or not the fighter identified them correctly in the first place.
Use magic items which cast Heat Metal
Depending on what skills and resources your spellcasters have access to, they could obtain or produce wands which cast Heat Metal. Doing so could go a long way towards hiding the caster, as the normal somatic and verbal elements would not happen. The wand could even be passed around, further obscuring which person cast the spell, and allowing fresh castings when necessary.
Having multiple wands, or some decoy wands, can combine this with the shell game approach.
Illusory decoy casters would make it harder to identify the correct target, even if the fighter knows which enemy initially cast the spell. These would have to be created and maintained by others, but since they are on hand anyways that's not a tall order. An extension of this could be granting invisibility to the one casting Heat Metal after they've cast the spell, via a spell maintained by someone else or a magic item, and would make them very difficult to locate.
Multiclassing and feats
With levels in Sorcerer, your casters can take the Subtle Spell metamagic feat and cast spells far less conspicuously than normal.