In Rivals matches you can collect Magatama. Some is scattered around the map and the rest is collected from defeating enemies. You get 1 from most enemies but stronger ones drop 5. At the starting area of the map are 4 alters where you can spend the Magatama you have earned. One is for a shade which costs 5 and there is also one for an elder shade that cost 15. Summon one will send an enemy to the other side to interfere with that teams progress. You cannot summon another until the previous one is defeated. Not sure if the elder shade counts as well but I think it's best to stick with the regular shade since it's cheaper. Just keep collecting Magatama and summoning shades everytime your opponent kills one. If they don't keep killing your shades, you won't be able to summon more. If your teammate keeps taking too much Magatama or you opponent doesn't kill them enough, you might have a bit of trouble. Took me a few tries. Cannot be done solo because there won't be an opponent to kill the shades.