It's not just Murdunks though. You have to do the three tasks in the frog area (one is an easy pick up 3 ground spawns and turn them in), and then the two that you have to kill various Rallosians and the named mobs by RCC ZI. Once you've done all 5 solo tasks + Murdunks, you can get a Tier 5 merc regardless of your progression in any of the other themes. The same holds true for Tier 4 mercs in FoS, Tier 3 in Kith, and Tier 2 in OGH.