Your best bet is to attack the ptolemaioi in Southern Africa - Ethiopa/Nubia area. That should draw their armies back down into Egypt, very slowly, and allow you to defeat them one by one. Then work your way up the Nile until you can sack Alexandira. Rather than fight them next to and around your homeland regions. Personally, I would advise only taking the Syrian cities after Egypt has been conquered. As for the Seleucids, dont antagonise them too much and hope they are distracted enough by the Parthians not to give you too hard a time.

Some players might advise you to just ship a powerful army straight to the Ptolies capital and destroy it.

Anyway, good luck and have fun!