I would say it is very premature to speak in terms of all-time greatness with Vasyl Lomachenko. Longevity is certainly a factor in determining who receives this type of label. He is an elite boxer of the current day who has great potential and could certainly achieve monumental things if he stays on the current course but based on all the criteria we use to determining all-time greatness, he is a good ways away from receiving that type of acclaim just yet.
But there still begs the question, how do you beat him? Outside of landing one single knockout blow, defeating Loma is going to be a monumental task for any fighter today. It would seem to me that it takes one of two kinds of fighters to beat him.
Assuming they could stay on track with a very disciplined game plan, it seems it would take a relentless but intelligent and aggressive fighter to make Loma extremely uncomfortable. Someone who could back him up while taking hard shots in return. A fighter who can let his hands go on the inside with constant moving of their head. Someone disciplined enough to not get bothered by the quick bursts and the elusiveness.
Think Roberto Duran beating Sugar Ray Leonard in 1980.
The other opponent who could conceivably beat him would be one who would be patient and sharp enough to stay at a distance and force Loma to come forward and leave himself vulnerable to the same types of counter-punches he likes to take advantage of opponents with. Someone with the speed and elusiveness of their own to force him to lead more than counter
Think Roy Jones and James Toney in 1994.