Well, I have to say, I can't think of a worse deus ex machina.I can. The Flash, season 2. They couldn't think of a good way to get rid of zoom so they had some time wraiths take him out....
But you probably have no idea what I'm talking about.
That wasn't nearly as bad. At the very least, it introduced Black Flash into the Arrowverse, which I'm sure Flash fans appreciated.
CW's The Flash
suffers in general from too many speedsters, and it gets confusing because they can all time travel and they can even make copies of themselves. Their powers are so OP that there really can't be anything other than a higher power (or another speedster) that stops them. It would be nice if season 4 figures out a way to make a compelling main villain who's not a speedster.
But that's neither here nor there; I could talk about Flash for hours, but we can't do that here :P For the Halo universe, changing history is about as lazy an ex machina as you can get.