It’s all about smoke and mirrors.
Trident Master Achievement Problem
It depends on what causes the fatal damage. If you’re applying conditions, and the enemy dies from a tick of condition damage, rather than direct damage from a trident skill, it won’t count toward the achievement.
Stitch has it right. If you really want this, you are best off killing ambients, which will die immediately and not give the conditions a chance to interfere with your score.