I would stop recruiting longbows and start recruiting crossbows. The AP damage really will make a difference against armoured units.
Of course you'll be able to recruit far fewer, which may be a problem if you don't have very many castles yet.
If you can either summon a storm (so you can cast 'summon storm power') or forge boosting items to get your magisters up to A3 you can stand at the back of the battlefield and cast Thunderstrike, which may not be enough to kill a Jotun outright, but will do a lot of damage (plus splash damage), while the giants run up the length of the battlefield to try to get to you. Combine this with crossbow fire (which will be most effective when they get close) and you'll cause them some grief.
Also at least some of your magisters should be able to cast Gifts from Heaven (after casting 'Summon Earthpower'), which is a great giant-killer.
If you have enough time to mount a long-term war I would start building castles so that you can start pouring out crossbows. And the spy thing that others are suggesting.
