Horse Rider - loose to a centaur while having Equestrian lvl 3, you'll be transported to isolated part of the map where you need to spend time with one of the centaurs and marry him/her

Queen - loose to a beast mistress while having Beastmaster lvl 3, slowly follow the conversation to fail all check including temptation one

Harpy Wife - loose to a harpy while having Cuckoo for Cuckoo lvl 3, she'll take you to her nest where you'll be both metaphorically and literally "fucked" for the rest of your life

Locked Up - check the wares of the strange merchant near first town while having Anal Addict lvl 3, same as with Queen slowly follow the conversation to loose all checks

Gokkun - go to the brothel and ask to spend time with one of the girls, ask for a blowjob, fail check and force her to do it. Repeat that couple of times and go to town center, fail check if needed and you'll be transported to another secluded part of map with castle where you'll spend rest of your life being a blowtoy for one of the brothel workers

Nursery - go to the abandoned, infested ruins on the way to town of monsters, I don't remember which or if all checks need to be failed with the spider lady so experiment a bit. However I'm positive that in order for the achievement to prock you must "leave the tiny spider alone". If everything will go well she'll pump you full of spider eggs and get you the achievement.

#1 Hero - defeat the goddess of mount xiuh on lvl 1, easier said than done obviously. I consider it hardest achievement in game so far, you're basically limited to skills and point you choose at the character creation since you're not allowed to lvl up so I'd advise completing all other achievement first to maximize the amount of bonus points. You can earn experience and gold to purchase better armor and items but DO NOT click on character/level up, as long as you do you don't mess up the achievement.

Just in case be warned that each of the scenarios above with exception of #1 Hero will get you the achievement but will also result in game over. To lvl up required skills you basically do the same thing which is loosing to a specific monster because below lvl 3 skill won't cause a game over.  Some exceptions are present like harpy that can be defeated and you can  plop on her afterwards increasing CfC skill with no danger of game over.

Tried other classes but not the mage in particular, I always find myself drawn to enchanter as I like to spread my points rather evenly and it has no real weak/strong sides allowing me to do so. If you say you can equip things like gauntlets or helmets with no problem maybe it's not in the game proper yet, or maybe you can't equip really heavy stuff like full plate.