You can try with Hydrazine hydrate with ethanol as a solvent in reflex condition for 10 hours..

It would be better to avoid primary and secondary alcohols as solvent, but tBuOH or ethers (like diglyme, dioxan, etc) might be useful.

I think it is not possible to synthesize the monohydrazide in large quantities as it is a intermediate form and the main product will be always the cyclic hydrazide.

See attachment

Depending on your use of this compound for further synthesis you maybe use pivaloylhydrazide

I think ,it is better to convert the anhydride to ester then add a suitable amount of hydrazine hydrate (not an excess as more procedures in literature) to get the mono hydrazide first,.

I think it is difficult to prepare the monohydrazide as the compound will have a tendency to form phthalylhydrazide (2,3-dihydrophthalazine-1,4-dione) on heating. I am not sure whether it will work or not, one can try the following procedure : Stirring of phthalic anhydride in large excess (say 10 times or more but without solvent) of hydrazine hydrate at room temperature (or at lower temperature) for a long time. It is expected to obtain the monohydrazide carboxylic acid salt of hyazine. Careful neutralization with with dil HCl under cold condition will generate the free acid and hydrazine will be removed as salt. Thus the monohydrazide will be precipitated. Filter it to collect the solid (excess of HCl to be avoided as it would make the hydrazide soluble as a salt). It is to be dried under vacuum without heating (heating can convert it to phthalylhydrazide again).

refer this artcle

Via reaction with hydrazin hydrate

Reaction with Hydrazine hydrate (equimolar) in EtOH at 50 oC under vigorous stirring condition may work best

I myself tried this few years back. Unfortunately, the cyclic hydrazide is formed irrespective of the reaction condition.