AG's have somewhat weak defense stats (at least compared to other heavy infantry), so pinning and flanking them is always a valid tactic. Make sure your "pinning" unit can take some punishment, though; otherwise they'll collapse before you can hit the Abyssinians in the flanks/rear.
As the others here have said, missile fire can also be effective against them. I find it's generally best to use some type of x-bow unit if you've got them (or Longbows if you're English), as the Guards' armour rating is still high enough that regular archers may not be very effective.
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