are there any achievements made by him?
He extended Mercian Supremacy over most of southern England and regained complete control of the southeast
Offa of Mercia
Taking advantage of instability in the kingdom of Kent to establish himself as overlord, Offa also controlled Sussex by 771, though his authority did not remain unchallenged in either territory. In the 780s he extended Mercian Supremacy over most of southern England, allying with Beorhtric of Wessex, who married Offa's daughter Eadburh, and regained complete control of the southeast. 
who is offa of mercia? King of Mercia, a kingdom of Anglo-Saxon England were there any coins bearing the image of the king? No. Coins carried his name. what is his stature? "rex Merciorium", "king of the Mercians and surrounding nations" and Rex Anglorum" how did he get the latter title? UNANSWERABLE
wiki:11603059
Offa of Mercia [SEP] Introduction
Offa had a dispute with the Bishop of Worcester, which was settled at the Council of Brentford in 781. Many surviving coins from Offa's reign carry elegant depictions of him, and the artistic quality of these images exceeds that of the contemporary Frankish coinage. Some of his coins carry images of his wife, Cynethryth—the only Anglo-Saxon queen ever depicted on a coin. Only three gold coins of Offa's have survived: one is a copy of an Abbasid dinar of 774 and carries Arabic text on one side, with "Offa Rex" on the other. The gold coins are of uncertain use but may have been struck to be used as alms or for gifts to Rome.