Thought-Stalker Warlock’s triggered ability cares whether an opponent lost life this turn, not how their life total changed. For example, an opponent who gained 2 life and lost 1 life in the same turn still lost life.

To choose a creature type, you must choose an existing creature type, such as Lizard or Wizard. You can’t choose multiple creature types, such as “Lizard Wizard.” Card types such as artifact can’t be chosen, nor can subtypes that aren’t creature types, such as Jace, Vehicle, or Treasure.