How long does it take you to grind to level 40? My guys are around the 30 mark and it's already feeling a bit tedious with the limited stamina.
Use your free yellow crystals to just get your Lv1 5 stars up to speed to around Lv15.
Then from there grind out the Training Tower. It took me about 3 days of casual play to get them to Lv40? I spent a lot of my time getting all my 4 stars to Lv20 and getting Anna and Sharon to 4 star Lv20 first. It's just making sure no one dies and everyone participates.
Healers have it easiest to level up.
Take advantage of the 1/2 stamina before they end it.