For Pontius Pirate, you can approach with one character, trigger the conversation/fight, use the teleporter pyramids to teleport back to the other character, and lay down oil, fire and smoke that opponents have to run through to get to you, with your characters behind a choke point. Summon(s) could also help spread them out by slowing some down and doing damage.
There are also explosive or poison barrels around, and a character can sneak in and add more, or grenades, to shoot once combat starts. With one character in conversation with Pontius, the other can target the other skeletons there and take them all out. Someone can sneak onto the boat and use the ballista there, but don't have your characters together, because it targets randomly.
I don't recall any particularly effective strategies for the other fights.
Grenades dropped, or drag and dropped, to the ground before a fight can be targeted after it starts, and an exploding grenade (firestome, nail, etc) can cause a chain reaction.