Complex sentence: The 9th Battalion furthest south should be attacking the artillery battery at Gaba Tepe and the 12th Battalion the reserve, with 26th Jacob's Mountain Battery who were to establish their gun line on the plateau.
Split into simpler sentences: The 9th Battalion further south still should be attacking the artillery battery at Gaba Tepe. The 12th Battalion were the reserve, while the 26th Jacob's Mountain Battery were to establish their gun line on the plateau.