Approaching galactic conquest as free to play

So looking at it from another angle, saying you must always focus on red crate, because it is theoretically possibly for anyone, is also horrible advice.
Since we don't know his roster or GP, I still think generally going for gold is the best time/frustration to reward ratio, and as such not bad advice. And the surest crate getting you the new character you can recommend, not knowing anything about OPs resources.

Anyways if OP wants to unlock conquest units he should look for videos, as others mentioned. I prefer kahzgul's guides on Reddit, since I like reading at my own pace. He always lists cheeses and feats that synergize, so really useful to keep the battles to a minimum.
And if you really want to unlock those units, be prepared to spend crystals on refreshes, so it's worthwhile to save crystals beforehand, if your income is limited.