On the level Grievous Intrigue there is a fight with General Grievous where you have to fight his magnaguards and force-throw objects at him to allow you to damage his hearts. If you go in freeplay and do this level, you can get your own characters to have Grievous’ grey hearts appear above their head. However this does mean you cannot continue with the level and have to go back to the hub to keep progressing.
I’m not sure how to actually get this to work but here’s how i accidentally did it.
Disclaimer: yes I did have secret codes activated including invincibility and every score multiplier.
In the fight before Greivous pushes you back into a control room of some sort by spinning his lightsabers and walking forward, blocking your path out.
I started by force-throwing objects at Grievous and hitting him until he has one heart. I don’t think it matters what character you use for this part.
I had my second character set as Grievous.
At this point my second character (Grievous) gained all four grey hearts above their head after i threw the fourth object at him. Enemy Grievous went down to one red heart but i couldnt reach him in time.
My secondary character then began the animation of spinning his lightsabers while pushing me back through the corridor.
Once in the control room, the door shuts until you get Grievous down to one red heart.
I couldn’t get out and continue the level because i couldn’t throw objects at my second character, meaning they kept the four grey hearts above their head.
i tried killing them but the just kept respawning with the grey hearts still there.
I tried switching to that character and i still had the grey hearts above my head (although my hearts in the top left were red).
I tried switching that character to another but whatever i changed it to just kept the grey hearts.
Even when i killed them as a different character they respawned with the grey hearts.
It is faint but in the images below you can see the real Grievous’ single remaining red heart far beyond the door (best visible in the first image).
This is when I was playing as Grievous with the grey hearts:
Meet boss Luxury Droid and boss R2-D2 (boss Jar Jar is the branding image):