Well... Try to put all towers in the same local, and flood the ground there with ice traps, in the first boss, I put 5 towers and +-6 traps in the same spot
just keep trying or look up on google how to finish the boss and if you reset your skills you can get all of them to money to build more towers using the extra money.
Put skill points on money (4) and gems (1, hoping to get ice & poison), and group all towers (boulder / spear) at the end of the path. Use ice traps to slow the boss down. That should do the trick.
@ranazzi: Are you playing another game??? with all skill in gold, killing all the first wave, I can build 2 rock, and 1 spear, and plant 2 ice+1 poison trap. HOW THE FCK you manage to get 2 rock + 4 spear, and mana for 4 ice+1 poison trap????????? AND still have 24 change left over?
build a mist tower just like in the video and on the other side of the path a boulder tower, all skill points on trapper and build 4 ice traps between those towers, wait until the boss is out of range, sell the towers and build boulder towers at the end of the path.