Complex sentence: Serbia defeated the Bulgarian Empire in the Battle of Velbazhd in 1330, becoming a military superpower during the middle part of the 14th century under the dynamic Stephen Uroš IV Dušan (1331--55), who created the Serbian Empire.
Split into simpler sentences: Serbia defeated the Bulgarian Empire in the Battle of Velbazhd in 1330. Serbia became a military superpower during the middle part of the 14th century under the dynamic Stephen Uroš IV Dušan (1331--55), who created the Serbian Empire.