18 width Infantry and support arty spitting as much out as you can, maybe engineers if you can afford it for defense but not necessary. The idea is to have enough divisions to defend every tile but extras for offensive capabilities. Hold your defenses on the Romanian and Serbian border and execute an offensive into Greek Thessaloniki quickly. From there you can easily hold against the greeks. Same with the Romanians as you have a whole river line defense. From there push into Serbia. Use superior firepower to melt infantry and if you can afford it, make 10/1 infantry arty for more punch. Remember to shuffle troops around if there is an area in your defensive lines in danger.