1 Good Claudius, that next was emperor,

Claudius > (Claudius I, 10 BC-54 AD, reigned 41-54 and visited
Britain in 43)

2 An army brought, and with him battle fought,

him > (Guiderus, son of Cymbeline)

3 In which the king was by a treacher

the king > [Guiderius] treacher > traitor (Hamo: see HRB 4.13)

4 Disguised slain, ere any thereof thought:

thought > realized [that the king was in danger and would be slain]

5 Yet ceased not the bloody fight for aught; 6 For Arviragus his brother's place supplied,

supplied > filled