[Taqwus]

Flaming arrows are considered magic weapons for the purposes of smacking ethereals IIRC, and the fire damage (separate!) is armor-piercing. It's a nasty spell.

With enough archers, you'll even hit somebody with an Air Shield-80.

'Fire at commanders' was a possible order in the first Dominions game, but was eventually removed -- it's both unrealistic (spotting the commanders over the throngs, during battle, is not necessarily easy unless it's a dragon commanding hoburgs -- and for that, there's 'fire at large monsters') and unbalancing (imagine R'lyeh illithid squads scripted to fire their always-hits range-100 mindblasts specifically at commanders, for instance, or Firbolg snipers with an eye of aiming for PREC-20, boots of quickness and a nasty xbow). Although... 'fire at mages' still exists, for a particular Troll Archer from what I hear.

Otherwise, anything fast -- especially anything that flies or tramples -- can help you get to the mages.