The Breviate Chronicle for the year 1091 notes:

kediuor filius gollerwin obiit Cuius filii inuitauerunt Grifinum filium maredut ? quem resus filius teudur expugnauit et occidit iuxta llandedoc (Gough-Cooper, b1113.2).

Translation: Cedifor ap Gollerwin died, and his sons summoned Gruffydd ap Maredudd, who Rhys ap Tewdwr conquered and killed near St Dogmael's (Remfry, 175).

The Brutiau expand slightly on this:

Ac y bu uarw Kediuor ap Gillwyn. A Llywelyn, y uap, a?e urodyr a wahawdassant Gruffud ap Maredud. Ac yn y erbyn yd ymladawd Rys ap Tewdwr yn y urwydyr ynn Llann Wdach; ac y gyrrawd ar ffo ac yd ymlidyawd ac y delis, ac yn y diwed y lladawd (Jones, 32).

Translation: And Cedifor ap Gollwyn dies. And Llywelyn, his son, and his brothers called in Gruffudd ap Maredudd. And against him fought Rhys ap Tewdwr in the battle at Llanwddach [Llandudoch]; and he drove him to flight and pursued and captured him, and at last he slew him (Jones, 33).

Llandudoch is the Welsh name for St Dogmaels (SN 162 460), but the exact site of the battle remains uncertain.

RCAHMW (Battlefields Inventory), Dec 2016

Bibliography
Gough-Cooper, Henry (ed.) The Breviate Chronicle: Annales Cambriae, The B Text from London, National Archives, MS E164/1, pp. 2?26, online edition.
Jones, Thomas (ed. and trans), Brut y Tywysogyon or the Chronicle of the Princes: Red Book of Hergest Version (Cardiff, University of Wales Press, 1955).
Remfry, Paul M. Annales Cambriae: A Translation of Harleian 3859: PRO E. 164/1: Cottonian Domitian, A1: Exeter Cathedral Library MS.3514 and MS Exchequer DB Neath, PRO E. 164/1 (Castle Studies Research, 2007).