We set out to make each encounter challenging and unique and our mini bosses are no exception. Our mini bosses come in many different shapes and sizes and each mini boss has its very own combat style. Instead of just increasing their health to an insane level we gave them new AI and specialized abilities, for example, an opponent might have the ability to regenerate or fly... It might even be able to teleport or if the opponent is beginning to lose a battle it might grab a better weapon and go into a more powerful fighting mode. I don't want to give too much away so I'll stop with that...
In some cases foes can use the map environment against the player, in other cases the player must use the environment against his foes... Then there are some shades of gray situations, maybe the alleyway you are walking down is heavily guarded so instead of trying to fight your way through, you might want to climb up on a rooftop so you can avoid your enemy but while you are up there you might find an object like a heavy barrel that you can drop on your opponent which will defeat him. Again, there are lots of different scenarios so keep a look out for such opportunities.
It's important not to forget your inventory items, which can be used in many creative ways, for example, instead of just trying to fight through an encounter it might be better to use your bow and arrows to create a sound diversion so you can simply pass through the area unseen and unheard...
It is worth reemphasizing that as a thief the purpose of your mission is not necessarily to defeat your opponents in melee combat but instead it is to succeed by meeting your mission's objectives, which usually involve stealing and retrieving valuable items.
The day is for honest men, the night for thieves.
~ Euripides (484 B.C. - 406 B.C.), Iphigenia in Tauris, circa 412 B.C.