I'm sorry but level one wizard towers at town hall 7-8 is not very good, same with mortars and cannons
I suck my town halls only level 5 right now but what you should always try to do is upgrade everything you have to its max before it requires further upgrade by the town hall first, that way you fully prepared to your best ability to fend off an attack.
I have not spent a penny on the game and I am currently on TH 7 saving up for the last Builder's Hut (2000 gems)