I really think you are bringing too much biased into this, there is no way too scale how powerful Rose's pokemon are compared to mega lucario, so if they were much weaker, then lucario beating them solo is not a feat? (even if you use pikachu vs Mlucario, it's inconsistent since latios and trip's snivy aren't comparable either). You also really underestimate the power of mega evolution in my opinion, that said, even if we compare grapploct to mega lucario, we never saw if lucario had battled a pokemon before he fought grapploct? and as short as it was, grappoloct had been sparring with poliwrath and pikachu that same day.
But my biggest issue is that you are comparing ash's lucario to dragonite vs mega lucario, but he also took on mienshao, and you haven't really included this in your comments. that brings the hurdle lucario has to overcome that much higher, since the latios/snivy example shows you can't just compare rose's pokemon to mega lucario in xy.