The basic idea is to use your Rangers in the center. You want them to have the range to hit either the top or the bottom lane, and the beginning and end of the middle lane, so you should place them roughly in the middle, horizontally.
Upgrade them as much as possible, as they'll be doing the bulk of your damage, and each upgrade means their range and damage increases, along with their skills. If you can get them high enough, it should be possible for them to hit the lane opposite the one they were placed to defend.
Try to put a monk behind them, too, to ensure they stay standing. If you can upgrade him twice, so he gets his area heal, that would help immensely.
Your melee units will be less useful in this fight; there's not many places to put them to make them useful for more than one lane. I'd suggest placing them close to the end, on the small spits right after the single center point. If you place them at those corners, that will maximize the damage they can do, and you can place one in the center spot, to ensure that the center doesn't go unchallenged without melee.
There are a couple spots to place the Ice Mage. Since you only have one, you can only place her to affect two lanes, at most. There are two strategies you can use: place her near the beginning, to allow more focused fire when enemies first spawn, or close to the end of the Ranger's range, to catch the stragglers that made it through the initial gauntlet.
Don't forget your Berserk spell; it can give you a couple seconds of extra damage when you need it. Use it often and early.
If it still doesn't work, try to get a few more archers, above all else. Their damage output at range far outweighs any other unit. After that, get another monk to keep them up. That should allow you to clear most levels without armored enemies.
