The Vita Griffini filius Conani (The Life of Grufudd ap Cynan) refers to Gruffudd ap Cynan sending `armed soldiers, (namely the sons of Merwydd)?, from his base at Abermenai:

'in Kelynauc et sanctuarium misit metu hominum Powisiae [?] ut Kynwricum oppressorem' (Russell, 60).
Translation: 'to the sanctuary of Clynnog out of fear of the men of Powys [?] in order to defeat the oppressor Cynwrig' (Russell, 61).

The text then goes on to say that they `killed him and a very large part of his men? (Russell, 60-1).

Merwydd ap Gollwyn was associated with the cantrefs of Eifionydd and Ardudwy. His three sons are named earlier in the Vita as Asser, Meirion and Gwrgan. Cynwrig ap Rhiwallon was the king of Powys (Russell, 60-1). The `sanctuary of Clynnog? refers to the church of Clynnog Fawr, on the Ll?n Peninsula (NPRN 43699, SH 414 496), but the exact site of the conflict is uncertain.

RCAHMW (Battlefields Inventory), Dec 2016

Bibliography
Russell, Paul (ed. and trans.), Vita Griffini Filii Conani: The Medieval Latin Life of Gruffudd ap Cynan (University of Wales Press, Cardiff, 2005).