He is still miles ahead better than farming with Leblanc. Her auto animation is terrible, and she has one of the worst base AD stats in the game, its extremely difficult to last hit with leblanc until you get enough ap to clear waves with W.
Anyways, the way you deal with the cannon minion is that you auto-attack it 3-5 times until its half dead, THEN use your E on it. Whenever your E jumps to a minion, you start auto attacking THAT minion to ensure it dies so that the E can jump again. repeat until the wave is gone. Eventually you will get enough AP to clear the entire wave without needing to auto attack.
One thing you should do on malzahar is build AP early. Do NOT rush mana regen items like chalice of harmony. You rush a blasting wand or if possible, a needlessly large rod. This will save you mana in the end because you will have to cast a LOT less spells to clear waves. (If you farm enough gold for a needlessly large rod, you will instantly start clearing waves with one E cast and it will refund all your mana). You should never use your Q to farm unless you ABSOLUTELY have to.
Instead of buying a chalice or rod of ages on malzahar, your first item should be rabadons deathcap or zhonyas if your up against an AD mid. Malzahar has some of the best ap-scaling spells in the game. Having more AP makes your spells more mana efficient, thus meaning you wont NEED as much mana. You are also 2x as scary to the enemy this way and they wont be able to harass you.