The trick here is to focus on one of them to get them to half health as soon as possible. They both begin summoning once one of them drops below 1/2 HP, and the summoning phase is your friend.
I find it best to focus on Jack first, since he requires much less focus to dodge. You need to attack Jack when the two of them are on opposite walls. If the two are on the same wall, Jack will always be in Will's line of fire, so it's not safe to attack him. But as long as the two aren't in adjacent positions, there should be somewhere where you can fire at Jack without getting punished. If you like, you can use magic to speed up the time it takes to get him to half HP. Light Magic deals a little more damage for its cost, so it's a good choice. Water is easy to hit with. Ice does the most damage very quickly, but is also very expensive. It's best to try to reserve a little magic for later.
Once either of them reaches half HP, they'll stop transforming into their human form for most of their attacks. However, hitting them with the Water Magic while they're in flame form will cause them to revert to human a moment later. Wait until you have a good angle on Jack, then hit him once with Water before hammering away at him with Wind.
During the summoning phase, you can get several free shots off as they stand there. The enemies they summon are the key to endurance in this fight; the summoned enemies have a very high life and magic drop rate, so it's worth it to hunt them down to restore your stuff.
The most dangerous part is when the two go into their walking phase. Try to keep your distance from Jack, while keeping in line with Will to avoid his X-pattern fireballs, and do your best not to get pinned into the corners. Also try to avoid the points along the walls where the two may teleport to after the phase is over.
Once Jack goes down, fighting Will is the same as before. He'll continue summoning even if you didn't get him to half HP yet. It's mostly a game of patience at this point. It's tough to hit him when he's in the corners, but Water Magic should do the trick. I find it easiest to hit him when he's walking around. Just keep in line with him and hit him from the sides, and you'll be safe from his projectile attack. Once he drops below half HP, use the same strategy, but douse him with Water to make him vulnerable when you need to.
If all else fails, it may be worth it to search for more powerups. Once you collect 20 coins, you'll get an upgrade that shows how many life and magic containers remain in each area, making it easier to hunt them down.
I hope this helps. If you have any other questions, let me know. 
