#1 is the established solution in many MMOs. The target gives a visual cue, like an animation, or a line of text (the former being the better choice, as you don't need to keep an eye on the chat window).

I would strongly advise against #2. It takes one or two attempts to learn the pattern, and then most of the challenge of the fight is gone.

This is something that's still very underused in other games, having monsters that actually change their attack pattern more often.