While Mega Evolution is a mechanic that ends your turn as soon as you use it, this Ability is a way to at least dish out some damage despite forfeiting your attack for the turn
Although, read my column first
Players hate to give you opportunities for card draw if they can help it the hatred aimed at stuff like Rhystic Study proves that , and pillowforting up with it, Ghostly Prison and Norn's Annex will at least buy you a few more turns or severely disadvantage opponents who try to get through
Once all those blocking creatures are assigned lethal damage, any excess damage is assigned as its controller chooses among those blocking creatures and the player, planeswalker, or battle the creature is attacking
Phyrexian Tower which can be tutored by Crop Rotation, Culling the Weak, Burnt Offering, Nature's Claim or bounce spells like Snapback and Suspend will get the job done
Being a dragon provides other synergies too for example using Crux of Fate for a one-sided board wipe