Orchid is not terribly hard. I recommend around lvl 10 for her, higher is better of course, but at 10 should be easily doable already. She doesn't do too much damage, but she does have lot of attacks that ramp up the lust points of your characters, and if they get to 90% lust she can perform a really high damage attack on them that is very likely to either kill them outright or leave them at really low health. So therefore the goal is to nuke her down as fast as possible before she has time to ramp up the lust.
Assuming you have Kia as healer (acolyte), first thing she does should be to cast the strength buff on Terry, and after that just buff the lowest health person with toughness or strength (note that multiple casts of same buff on same target wont stack), when nobody is badly hurt, and otherwise heal whoever is hurt. Terry, assuming he is rogue, should always keep swift buff up, and rest of the time alternate bash (bash is in the skills section for him from the warrior levels he starts with) and dirty blow to both do high damage on Orchid and stun her to lower the amount of actions she gets. Then what to do with your main character depends on your build, but overall if possible use abilities that stun/slow Orchid (like Distract from courtesan, and Bash from warrior). Also if your main character is scholar, then open with Cheer (the toughness buff from Kia does not stack with Cheer though), it helps a lot in the survivability front.
Now if you are still struggling, then you could purchase a glass sword (you also get one as quest reward from completing Vaughn's quests in the outlaw camp) and have your highest damage character (probably Terry) use that on Orchid. Its a single use item but does massive damage. You shouldn't really need to use glass sword on her though and they are fairly expensive so would recommend saving those for the harder bosses later on.