A double-sleeved EDH deck is probably too tall for most players to shuffle comfortably. You may want to consider splitting the deck into halves and shuffling each ~50 portion individually before recombining them.

When mash shuffling, in order to prevent creased sleeve edges, it's best to use a gentle and precise shuffle. Careless and forceful shuffles will cause sleeve corners to catch and bent. After taking one part of the deck (or half-deck) you are shuffling in each hand, when inserting the bottom corners of one part into the other to initiate the mash shuffle, do it as carefully and precisely as you can without being too slow about it. Once the halves have "locked" in place and the corners have fully intersected, let the top half quickly and smoothly fall in place via gravity, there is no need to force the mash after this point except to neaten the final product.

Finally, purchasing more rigid and durable sleeves such as Dragon Shield Mattes will also help prevent corner creasing, but even Dragon Shields will crease if you shuffle too violently.