My provinces keep getting the waterloo treatment

[Endoperez]

You might want to mix the Bandar Swordsmen with smaller apes. If you had 3-4 markata for each Bandar Swordsman, and they mixed evenly, each square would have 2 Markata and 1 Bandar Swordsman (instad of 2 Bandar Swordsmen). When a giant strikes at a square, he'd have 2/3 chance of hitting a markata instead of a Bandar, and the Markata would have a better chance of surviving the hit due to his defence of 14.

In this case, numbers are more important than skill, and Bucklers won't help as much as usually since the giants' incredible strength often allows them to deal damage even when their attack is parried, so you might want to try Bandar warriors with Cudgels.

Also, remember to use your mages. After some Thaumaturgy research, Gurus can cast Mind Burn (which helps but isn't very good against giants) and then Paralyze. Each Paralyze that gets through the magic resistance of a Jotun effectively kills the giant, as long as your units aren't defeated before they manage to kill the paralyzed one.