In Fantastic Beasts: The Crimes of Grindelwald, how can Credence cast such a powerful spell when he receives his first wand from Grindewald?
We don't know for sure if he actually cast a spell.
We never saw him muttering any spell and for a wizard to cast a non-verbal spell, he'd have to be way way experienced (like McGonagall + level experienced). Also Credence doesn't know any spells yet. He has only raw power.
So here's what happened.
Just like Harry did in Harry Potter and Philosopher/Sorcerer's Stone at Ollivanders while selecting his first wand. When Harry was given his first wands, he just swished it around and a vase breaks. When he's given his second wand, he swishes it around and all the wands fall off the shelves.
So I'm assuming it's something like this for Credence. Given that he's an Obscurial who lived to 20 making him extremely powerful (max lifespan of an Obscurial is 10 years and he is powerful enough to kill Albus Dumbledore). So a first wand in the hand of an extremely powerful Obscurial caused an extremely powerful reaction just like Harry's did in Part 1.
So it's just that the wand reacted in a very powerful way to Credence's flick of the wand for the first time in his hand. It's a first contact with the wand and his raw power and his intent. It's not any powerful spell that is used.