Arman can't beat him.
Only Islam can beat Topuria in this division, since he's a threat in both striking and wrestling. This makes it almost impossible for Topuria to put his usual pressure on Islam if he keeps his distance with the reach advantage, and very VERY dangerous kicks, plus cage wrestling.
Arman's only threat is wrestling, and I'm pretty sure Topuria can adapt to that very well.
It's a shame, but we'll probably never see it. Islam will most likely never return to LW, and we'll have to watch joke fights like Topuria vs. Pimblett because the UFC is a complete joke.
But Luffy vs Topuria can be a banger tho, Luffy is awesome, but still have to see how he deal with pressures strikers.