Air magic you say? (well yeah duh... its MAN!! or something like it), how about ORB LIGHTNING? I mean you're gonna research it anyway so why not use it?
Or alternatively you could plan on trying to use flamming arrows, though that's probably a bit more difficult unless you want to use a special pretender to generate the effect.
Pack enough longbows (and ignore the knights, just get some fodder to slow the enemy down...) and wind guide and you'll slowly devistate even the best normal infantry, its just a matter of patience, oh and sticking the longbows all the way in the back
Hell I've done that with Caelum and their cruddy short bows
Re: Dealing with high protection
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