Level 1 - try to trade summoners with your flash + ignite + glacial/aftershock proc + insane early Q damage + passive if necessary + biscuit. For this to work you have to hit both parts of the Q and it usually sets you ahead since no one expects this. If wave too big or they have lethal tempo/too much dmg chill but most of the times even if all that happened was a trade in summoners youll get them back faster with cosmic.

Level 6 - You have insane damage and insane heal, look to jump unto lane or if they are playing safe just roam and watch out for anyone under towers, you can jump from out of vision and push them into tower with E, then push them into tower with ult, then push them back with Q, then everfrost them under tower if you have it.

Level 11 - RUSH THIS IF YOU'RE CLOSE TO IT, you heal an extra 1% per blob but with how many you make it is a loooooooot, its a bit tank powerspike and youll survive way longer by just being lvl 11.

Buying dark seal is good at any point in the game since zac can get so many assists and W dmg is both spamable AND easy to stack with dark seal but ofc its fine if you dont wanna go for it as its still risk-reward.
Cooldown reduction is extremely good on zac if you wanna tank since less cd means more blobs which reduce W cd even more with means even more blobs. Sometimes its okay to just stop attacking to go back and pick up blobs.