I find it easiest to build a trap using 4-5 behomoth gateways (arranged parallel to eachother in a “rib cage” like formation with 2 gates on either end, leave a little space between just enough to slip through with an Icthy or Megalodon) in order to tame a Mosa, simply leave one side open to lure the Mosa through before speeding behind it and shutting it in.

Bring lots of bolts and/or arrows and a healthy amount of narcotics because its topor depletes fast.