does this mean,
"im going to be doing it"

i think that "yapıyor olacağım" can also be used..?
which one is more common?
are there any differences??

yapmış olacagım * i would have done

yapiyor olacagım * i will be doing

Both are uncommon. These kind sentences don't have a wide range of use. Rarely this structure is used maybe in such a sentence:
"Sen geldiğinde ben gitmiş olacağım".

Since it is common to see the structure "verb + olacağım" (or any version of it) in translations it doesnt mean it is common in Turkish.

so what is more common? what would be used instead of that?

The sentence "I'm going to be doing it" sounds unusual to me as an English sentence. I have replaced it with the following:

"I'm going to do it" : Bunu yapacağım.

"verb + olmak" forms are used to express an action in the future that is preciding another action. In other words, it expresses past in the future. In English we do it with future perfect tenses. Both in English and in Turkish past in the Future are uncommon.

Future Perfect Tense: will + have + past participle
Example:
I will have written the report before you arrive.
Sen gelmeden önce ben yazıyı bitirmiş olacağım.

Future Perfect Continuous Tense:
will + have + been + present participle

When this program finishes I will have been wathing TV for eight hours.
Bu program bittiğinde sekiz saattir televizyon izliyor olacağım.