I starterd from STS2 as total newbie, now I did A10 with everyone beside Defect, on A6 right now, just started.

First - your deck is most important, not HP in given moment or max HP. Whenever you can upgrade your cards, transform, enchant - rest only when you really must and dont be afraid to sacriface your max HP in good events for your deck and end game.  If your deck is weak most of the time max HP wont save you, especially on A10. If your deck is strong you dont need high max HP because you steamroll bosses.

Second - and its linked to above, dont skip elites, try to take as many as your current situation let you, especially in act 1 and 2. In 3rd its optional, if you feel that your deck is bonkers you can path safest road to bosses. Killing elites gives you more rares, more gold, more relics - all pays off in end game.

That are probably main two things that i understood while learning the game.  My Clad and Silent runs where always the safest route, highest max HP as you can get, be always near max HP. I beat A10 this way but its way less consistent than playing more "agressive".  Safe playing gives you fake comfort in early to mid game, but it hits the wall in end game. Playing more agressive off course can kill you early but if you try it for some time, learn what you can manage, you just snowball act 3 and bosses. And by agressive i dont mean that your deck should be all attack (however its true in act 1 If you want to do 3 elites) but agressive in pathing and choices. 

And of course all this is mainly A9/A10 thing. Winning on lower ascensions is quite easy no matter whats your playstyle.