Without giving a crash course in Thermodynamics, and Physics, fully 60% of air conditioning is used for humidity control(removal of latent heat), the rest is just to remove "sensible" heat(make cold). The ideal settings are to *MAINTAIN* 75 degrees at 50% RH in the conditioned space. If you get the conditioned space down to 68 degrees or lower, the evaporator coil is gonna freeze....I can't change the laws of Physics. Same if the temp in the unconditioned space(where the heat is being moved to) drops below 60 degrees unless it has head pressure controls that are mostly found on large commercial systems. All this being said, Its better to go with a little bit smaller system or unit than what you think you will need(it helps to do the heat load calcs) for himidity control. Also keep in mind that with the humidity removal, you will have to water your grow a little more frequently.
However with A/C, there is an added bonus.... In a closely controlled space that maintains 75 degrees at 50% RH, the incidence of molds and such will be reduced to almost nothing.

Can you guess what I do for a living?? LOL