Penguins are to raise levels, when people say "fodder" that usually means promotion material for higher star levels. You can't use penguins to promote units.
You probably know this, but maybe this helps some other newbies to split this up again:
The chain is mostly this:
2* "fodder" heroes -> use them to promote 2* Phantasms to 3* OR MORE IMPORTANTLY level them up to level 20, then use other 2* to promote them to 3*, and use them to promote 3* Phantasms to 4*!
2* Phantasms (silver) -> Phantasms come at max level, ready to promote! They also gain more XP than normal units so they are easier to level!
Use 2x 2* fodder heroes to make them 3*, then level them to level 30 to use 3x 3* fodders to make them 4*.
3* Phantasms (gold) -> Use 3x 3* fodder (or 3x 3* silver phantasms if you're impatient and/or have stigma lying around) to promote them to 4*. Level them to level 40, then use 3x 4* Phantasms (silver) to make them 5*! Bam, 1/5th of a 6* unit promotion done!
4* Phantasms (black) -> yay, no need to level these! Use 4x 4* Phantasms (silver, or gold if you are REALLY impatient) to make them 5*, easy.
For leveling any 2* fodder or any Phantasms, penguins are of course the easiest way. Using penguins costs gold, but no stamina, so it can be the preferred option. It's also quicker, duh.
The long and winding road to level them, is making teams that can drag a fodder or Phantasma along to clear anything you normally do, like lower Spirit Altar levels if you grind for runes anyway, or adventure stages when grinding for catalysts you need.
There are even decidated farming spots that people recommend to maximize XP gain per stamina (some of the labyrinth like adventure stages for example, because you defeat more monsters = gain more battle XP for the same stamina cost) if you want to focus on leveling promotion fodder.
Here's a reddit thread with a breakdown on XP gain and how it changes depending in how many maxed units you run:
https://www.reddit.com/r/EpicSeven/comments/a34xlp/math_exp_share_leveling_efficiency/
