Ascend 10

Complete Ascension Level 10 with any single character. Each Ascension level stacks another nasty modifier (tougher elites, less healing, weaker starting relic, and so on) and you climb one level per win on that character - so this is roughly ten wins on one class, each a notch harder. You do not need the true ending; a standard Act 3 win at Ascension 10 counts. Pick your comfiest character and grind it up. Halfway house to Ascend 20.