With an Archer, you can shoot at two different enemies, just like with any other unit with two attacks. And two shots are much more useful than one – if you miss once, you'll definitely hit the second time, but with one shot, you miss and that's it. Although, based on my own preferences, I choose the Ice Guard, but I admit that the Marauder is better.
I already wrote about the composition of the squad in the discussion thread about my favorite race, but I'll repeat it: Hero-Warrior (even if weak, but I don't see the point in taking a Mage or Archer – there are plenty of castle units for the second line, but there's nothing to cover them), two Plate Mails, two Ice Guards (water attack eliminates the need to take a Mage against creatures with physical immunity), and the last Sylph – a healer with air protection.

