(SPOILERS) Question about COG - relating to Grindlewalds self perceived power vs Dumbledore

When watching the film i recall Grindlewald saying to Credance that he is the only entity who can kill Dumbldore. I thought when i heard this that it was Grindlewald admitting that he thought Dumbledore would beat him in a duel... but later we find out they made a blood pact. So perhaps he was counting himself out due to that.. but blood pacts can apparently 'possibly' (to Dumbledore's knowledge) be destroyed, surely Grindlewald knows this.

I've only watched the movie once, i was a bit confused in regards to the neck-lass, I'm assuming it will be needed to destroy the pact which raises the obvious point that Grindlewald seems to be protecting it... of which makes one think yes, he does fear fighting Dumbledore. He clearly doesn't love him enough to not want him dead, why not do it himself? What i took from the movie is Grindlewald thinks Dumbledore is more powerful, of course he would know better then anyone with them being so close when they were younger.

Definitely a change from the over confidence Voldemort who would never display any kind of self inferiority towards anyone. I would have thought he would atleast give himself a shot at beating him. Really puts Dumbledore's power into perspective.