I agree that battles in swamp are a pain. I also think that’s exactly how they should be! Game is not supposed to always give you perfect conditions, just as you wouldn’t get them in RL. You don’t get convenient paths through every bog, sometimes you have no choice but to go waist-deep in the muck.
You can either manouver around it (lure enemy to different terrain before engaging as your band moves faster than others, utilise Pathfinder to lessen the impact), use it to your advantage (use a lot of ranged weaponry and make enemies into pincushions as they trudge towards you at a snail’s pace) or just avoid swamps altogether if you’d rather not get through the trouble.
It’s said plainly – not every contract is worth the risk. You can abandon contracts. You are not obliged to raid every swamp-based location. Make your choice.
As for a bit of advice I tend not to move around swamps too much – equip front bros with kite shields so they can weather enemy bombardment while giving all the others bows and crossbows to out-shoot opposition. By the time enemies are in melee range half of them are dead and others are tired, injured and heavily demoralised (I recommend both Fearsome and Crippling Strikes on your rangers)
Good luck! :)