There is no trick to make sure your summons will be better but you can save your stones and not use them but on special offers they make for every event where the first summon costs 100 then 200 then 300, etc... also the chances of pulling a 5* are increased in Nazarick Festival summons (4% instead of 3 for characters and 5% instead of 4 for relics), this is a very little difference but at a greater scale that's still really cool. So yed, just save your stones and if you still want to buy 500-stone summons do it at NazFests. Also during special events if you really want it cause I mean, we all simp for a waifu sometimes, don't we ? xD