I try to take belgrade on the first turn, if that fails, i usually send the additional units via event to galicia, and go on the defensive in serbia, as even with entente suppport its not easy to break through. Sometimes german support arrives to help push through montenegro or any Valjevo to belgrade or the Uzice route.
In Galicia, with additional units i defend in lemberg, and try to endanger Radom and Ivangorod if possible. Well placed counterattacks are needed, and german distractions in Prussia or Western poland. Without the additional units, i go to Przemyśl in a fighting retreat to allow my units to mobilize safely on the border. Attacks towards ivangorod can be considered later, and if poland falls a rush to kiev is always fun.
Navy has to contest the port of Serbian allies with subs and mines, maritime bombers should be placed from both germany and austria to support it. Also, german subs should always be built here if given an option, you will have an overabundance of them either way. Other tasks are naval bombarding the italian front, it greatly influences the war, and naval invaions. Your best targets are: Corfu, to give you better support against being blocked in the Adriatic, Bari-Taranto Region to divert italian resources, and open a new front when youre ready, Ancona if the situation is good enough for push to Rome, and sometimes landing in albania/greece is possible.
Italian fleet must be relentlessly pushed out of adriatic, mines are sometimes useless in small waters because of air recon, Naval weaponry is always a good upgrade and can suprise enemies. Naval warfare is useful for morale, but mine limit shouldnt help much against human opponent in this theatre like i said.
Entente can also flood Adriatic with submarines, and you have 1 DD in 1914 with 1 in production (not enough, even with air support), which is why ASW is very useful for A-H, in contrast to the germans. Also a Seaplane tender is not the most expensive unit, and can help against subs or harbours aswell.
Diplomacy for all CP should at first concentrate on getting bulgaria to join, its benefits are huge, and you can help the Ottomans to prevent a domino effect of Ottoss falling then A-H with the convoy support. Romania can be dangerous, but if you have bulgaria in the war, usually wont be a problem.
Techs to prioritize are trench warfare, infantry weapons, inf warfare, fighter development as usual, but depending on the course of war you have to prioritize between those: ASW, Naval weapons, Submarines(forgot the exact name), long range aircraft, Industrial development, ampphibious warfare(its actually not necessary but helps), logistics(to uphold the beachheads in harbours) and thats about what i used.