Frontline is very flexible: Lillia + Amumu + Sentinel are your flex options. Best with Lunar emblem so you can avoid the Alune lottery. When in doubt, add frontline to stall for Eclipse. Lux caps the board.

Stage 2: Play any tempo opener that can lead to Aphelios. Standard board is AD + upgraded frontline. Take the spatula off carousel for Lunar if possible.

Stage 3: Hold copies of Solar on your bench. If you upgrade them, play the Solar units as frontline. Kayle is quite weak so don't carry unless necessary.

Stage 4: Level to 8 and look for any frontline that can stall for Eclipse. Lillia 2 is the easiest to hit, but you can also tank Amumu, Sentinel, etc.