Always pleased to have a fan.
The achievements aren't necessarily the best explained... I guess that's what makes getting them an achievement? Really though, I don't like frustration in my players, Titty monster requires Alex to give you a titjob in the forest as well as a scene that happens where Alex talks about how Sami apologised to her, one of the options there involves her attacking you as the titty monster. Both of these combined in one run will give you the achievement.
Clothes that fit and Lest we forget are both results of giving Alex some of the milk Haily can send you. You get the second one if you also give her a chest enhancing massage.