For a great account of how to fight Varangian Guards, check out Alfred Duggan's novel "Count Bohemond." Bohemond's dad, Robert Hauteville, led a band of Apulians and Normans into Romania and fought it out with the Byzantines for a number of years. In one battle they came up against the Emperor's elite Varangians and discovered that their cavalry would not charge against the massed ranks of axemen - the horses would baulk at the last minute.

The situation was saved by two factors. Firstly, Hauteville's wife had joined the knights and managed to rally them (basically, they wouldn't rout while a woman was still fighting). Secondly, Bohemond called up his mercenary crossbowmen and promised them the golden ornaments belonging to every guardsman they could shoot. The Varangians loved their bling.

So combination of AP shooting and hit-and-run cavalry attacks would seem to be the effective tactic. It worked for the Apulians, who slaughtered the Varangians to a man.

Henry Treece's "Horned Helmet" also contains a brief (fictional?) account of a battle between the Jomsvikings and the Patzinaks wherein the elite Vikings are slowly ground down by an enemy with whom they cannot get to grips. Mounted archers can play havoc with any non-missile infantry, especially if they outnumber them.