How can I always land a lobber EX for legacy weapon swap glitch? is it like timing practice or there is something specific i need to do?

It's a matter of timing.

If you mean that you don't fire the EX, I understand that it happens when you change the weapon before the EX comes out (it's a nothing), so you have to time it well to let the EX come out and then you change weapons.

i meant that i need to always do the LOBBER ex (more damage most cases)

Although this way not be the most optimal strategy this is what I do and it works pretty well. When I ex I look at what weapon I am on in the bottom left of my screen. If it’s lobber, either don’t swap or swap twice super quickly depending on your confidence level. If it’s roundabout, I swap once. Like I said this may not be the absolute best way of doing it but it’s what I find comfortable