Mega Tonberry

[Guslington]

What should I do to get him at the Bevelle underground?

[Serpentine]

When you get to the lower levels like 40 then he will appear randomly on the map like the normal tonberry or greater drake.

[Guslington]

What should I do to beat him ? My team's level is 90 and I have Rikku wearing Berserker with auto-regen, auto-wall and auto-haste abilities but it doesnt seem to help much ! with only a stab he can easily knock them down and the damage is massive ..so what tactic do you use beat him?

[Serpentine]

One of the easiest ways to do damage is with the cat nip accessory (That does 9999 every hit when hp is low) then use trigger happy and boom you have lionheart.
Also even though 90 is high them low level are much easier at 99.
I use darknight berserker gunner yuna's special dressphere the great whirl move is very powerful.
If you want to make it plain easy get an episode clear in all places except bevelle to get moogle and then you should no problem.
I prefer the cat nip though