Split AI's army by making him chase a fast unit like camel or sphinx.
Use your defenses to guide AIs army around your towers.. while AI units chase me they get hammered by towers without ever getting in range of my villies ..
then when u have maxed your population and most techs are researched drop a siege unit close to his outmost town center .. by the time AI reacts his town is toast and his max population cap is reduced .. AI will send a villie to rebuild but park a unit or two there to keep harassing that villie until your army arrives - then with my pop 56 and his 130 at one point - i can beat AI at titan level !
You have to know which units counter AIs units best e.g. turma will toast slinger so distract it using a spearman which has best arrow defense while my slingers slowly toast the turma since they are best eggy unit against turma unless you can mass 2-3 sphinxs which eat turma like dessert.