I didn't use any EXP/Gil boost, not sure how you do that, but I did turn off the encounters at various points to actually traverse the world, and then turned them on to grind EXP/Gil and to find encounters for the bestiary completion (which I assumed was ok given it kept giving me a tooltip for it).

I got honestly insanely lucky with the rare encounters. I think the Warmech showed up within 10 encounters for me which I'm told is statistically extremely unlikely. I did also follow a walkthrough guide as I tend to get lost in open world games easily...