It's not as hard as you think and it is foolproof. The only qualification to get to level 80 is xp. Once you get the required amount, you are level 80.
PLaYING at level 80 is a whole new ball game. For that you need 750 IL items, at least Rank 10 runes and enchants, and a Gear score closer to 18k than 4k, or even 10-13 k. You will have the option to get up to 980 IL armor pieces outside halaster's Lair (The Mad Mage).
You can level up by going back and finishing many the zones you skipped on the way to level 70, the xp adds up.
Those who think they can solo Undermountain in a couple of days are going to be given a rude awakening by the level 71 Spiders on the Catacombs level.
Practice by doing The Frozen Heart solo, the monsters are level 73 Dungeon Dwellers (they have more HP and damage dealt, think 1-shot kills, than regular level 73s) If you can't kill the first group without dying 10 times , then use the alternate plan above to get to level 80. All my alts are 13k or less and this is how they will be getting to level 80.