Complex sentence: The Byzantine vanguard concentrated their attack on the centre where the Bulgarian leaders were positioned, but once the two main forces met and hand to hand combat ensued, the Bulgarians stationed on the heights showered the Byzantine force below with rocks and arrows.
Split into simpler sentences: The Byzantine vanguard concentrated their attack on the centre where the Bulgarian leaders were positioned. Once the two forces met, and hand to hand combat had begun, Bulgarians stationed on the heights showered the Byzantine force below with rocks and Arrows.