Good point, Tremir. A mixed group (melee and metatherics characters) is usually best in the long run, much better than one of both exclusively.
If you still have to attack a firecoated enemy using melee, try using slow, but harsh attacks, like the double-blade and double-blunt. Those, while they may take more AP, deal more damage as well, usually, and if you can do the ca. 160 (100 health and 60 defense) damage with four strikes, you’ll take half as much damage from firecoat than if you need eight strikes. (Firecoat always deals it’s damage value directly to the health of a melee attacker, at every single strike. Less strikes = less damage.)
Another good option is Strengthening your melee fighters, or weakening their target, or giving feedback to your target. Strengthen will increase melee damage, weaken will increase melee damage as well (yours, that is) and feedback will reduce the value of firecoat – beware, though, that the latter effect only starts when it is re-applied (so a firecoat (5) will stay a firecoat (5) if you hit the enemy with feedback. Once it runs out and has to be reapplied, however, it’ll likely be lower.)
Finally, Firecoat needs AP as well. Anything that has an enemy lose AP (such as Grasping Vines (Plant) or Pressure Point Punch (your Field Surgeon is able to do that, I think)) will force that enemy to select EITHER reapplying Firecoat once it runs out OR attacking one of your people with an actual attack. Usually it is enough to block one or two enemies out of a group, as this will lead to someone being not firecoated. Use that opportunity then, and strike him down!
