My man Zac is one of the best engage supports ever, he is even more deadly with a good adc, and his late game potential is huge. I for one 1v1 enemies Mid-Late game with my full combo.

Zac combos go a little like this.

Normal engage:
E+Q+R with W Spam during R duration

Other combos:

Q (to draw them in, if they are a champ that can dodge your E) followed by a E+R and W spam

Q (to draw them in) and then flash not only towards them BUT past them so that you can press R to push them TOWARDS your allies (Because Zac's R pushes people away from him), this is a good combo for when the enemy is under tower or refusing to come close enough for a fight.

E and then while they are knocked up, walk past them and press R so you can push them towards your allies. This is a good way to save flash IF you are confident the enemy cannot dodge your E.

In early game, it is a good idea to hide in bushes to make them anxious, and only engage after you at least level up to 2 and can use your E+Q combo, only do this if you are confident that they will die because at level 2 you are super denfenseless after you use your abilities, at level 3 you can at least spam your W for goo to heal yourself as you retreat.

Mid to late game what you'd like to be doing is hiding in bushes near your allies and being ready to jump in for a combo, you can also if you'd like, 1v1 a weak enemy, zac can do 3.2k damage with his full combo at level 18 (that doesn't include basic attacks), so its fair to say that you are both a tanky support but also a good damage dealer that can still heal mid-fight.