Rofl, just because it's on youtube that they use gambits does not mean that it is impossible to do it without.
First of all, it isn't 'Yizamat', it's 'Yiazmat.' I remember my fight with Yiazmat very well, and I did it completely without gambits. And, it lasted for well over an hour. Gambits don't do anything that you can't do yourself. They just make it easier for you to just use only one character. Like I said though, they have a slow reaction time in some cases and I just didn't like that. For example: the person in the video shown above was level 99 when they defeated Yiazmat. I wasn't. It was incredibly difficult, let me tell you that, but it wasn't impossible.
I know that I also did the same for the other three you listed: Omega, Phoenix and Hell Wyrm; but Yiazmat stands out the most in my mind. It truly was one helluva epic battle.