The conditions are that you need a chunky creature and that youre hitting a smaller one, but thats not hard for Gruul particularly with so many cheap and pivotal creatures being deployed to curve out
With those conditions met you put the untap spell on the stack, tap Stella to copy it targeting herself, let the copy resolve (untapping her), then use her ability again on the original spell to reset the loop
There are two phases to the game against Mono-Red: dont die and kill them. Your goal is to sideboard into all threats that are good at starting off as dont die and rapidly turn into kill them. Planeswalkers dont do that, and Jadelight Ranger usually just gets sent packing by Abrade or Ahn-Crop Crasher
Five mana is a bit much to pay, but the potential to get both modes on this lieutenant card is a very real advantage
His conditional indestructibility has the potential to turn off kill cards from decks that dont have early drops or cant keep them on the table, and his recursive potential is unlocked by any deck that either a) plans to use Delve (Tasigur, Angler, Treasure Cruise, Dig Through Time) or b) plans to abuse Deathmist Raptor and Den Protector
Beyond all that, it's also just awesome to watch every enter the battlefield effect on your side of the table trigger simultaneously
