You have to depend on the Companion in World content?

Set it on "decide what you want to do yourself", draw the mobs and keep the forward AoEs aimed at you. The Chocobo Companion will cast regens or heals if necessary, and you can move the mobs so that their frontal attacks do not affect it. (Think "I am the tank").

It already costs anywhere from 1 to 1.5 million gil in Onions to level it to 20.

For those actual Tanks out there: why do you want the Chocobo to be that DPS who pulls instead of you? You want it to attack first? Use 'Sic'. Just don't put it into "do what you want" mode, or it will think you want it to tank, which is not what you really want to do.