Try to focus on upgrading Epics including their versions of these units. Upgrading epics are faster and cheaper. Plus, epics drop more than legendaries, allowing you to get them past 9 more easily.
Once you have a sold team of non legendary units, you can upgrade legendaries if you want. But working on legendaries before than is a hard way to go.
A good guide for what to upgrade is the unit requirements to do Insane Campaign. You can find the entire list in the forum:
By the time you get that done, you'll have a great base team of units and upgrading legendaries (or maybe even mythical if you get any by then) will be easier to do then.