Try my combo:
6 Speartons with shieldwalls+bash
2 Archidons/1 Albow with upgrades(Optional)
2 Magikills with full spells.
3 Merics with cure(must have).
3 Giants full-grown.
I beat the Marrokai level and Medusa level with this combo.
Also, there is middle tower in the campaign, check it yourself.
Of course, you need a lot of money(around 10,000 Gold and 2,000 Mana) to make it, so here's how:
1.Use your gift units(free units given by the game) to control the middle tower.
2.Hold it as long as possible, while creating my combo.
3.If you succeed, you should prepare some walls and castle archers(back up plan)
4.If you fail.....you just have a bad macroing skills.
P.S: Usually I have 11 miners (8 gold, 3 mana), and then sacrifice 2 of them for 1 Albow or 2 Archers.