Fighting Commanders and Equipment.

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As a rule of thumb, don't spend precious gems to equip non-mages commanders with less than say 30hp.
As exceptions you can make some "assassins" out of anything with a good prec and Eth Crossbow or Black Bow, or makes some supporters with Bow of Wars or Skeleton amulets, but limit investments in these to 1 item (2 at most with Eye of Precision).