Its ALREADY harder than it used to be to get 5 stacks (seriously, he has to stick to you for quite a while now to do it; unless you're already low, then he can just dunk you and hope his ult actually executes you without a teammate taking it or something stupid like you surviving on 2 hp)
If he enters a bloodrage, kite him until it ends (or just blow any kind of mobility spell since Darius won't be able to really catch you at that point lol). Its supposed to be a window of power, where he becomes absurdly strong, and then loses it after a short while (think Ryze's passive), you're not supposed to engage darius while its active, thats like tower diving malzahar and wondering why he just obliterates you without taking a single hit.
OT: That all said, I don't like the heal on Q, as someone who played a ton of Darius pre-rework, it just feels out of place. Personally I'd just remove the heal, and boost the damage on Q a bit more in exchange (the heal also helped to mitigate one of Darius' bigger weaknesses, being gank fodder)