the less reliant on squishy damage dealer you are, the faster you can scale up to POTD (caster are quite fragile, if you are slightly careless they can die... making the fight much much much harder - not so much with monks or rangers) - thats my experience with PoE, i played <POTD maybe 3 times (runs that lasted >1h), my first run ended in gilded vale after i lost my grimoire to a bug, my 2nd run ended to redric castle bug, my 3rd run ended in act 2 because the fighting was too easy (the AI was more "oh lets focus the tank" then)
the by far worst run i had was with a mage MC (okay, 3 con mage... wasnt the best decision i ever made)... i was raging like mad at the end of the run - sometimes i had to kill him myself so he wouldnt regen the last 3 endurance (else he would have really died)... i didnt even include him in every fight (more like... i used withdraw after 1-2 spells cast, if i could have benched him i would have done that)! (in one fight in the endless path i had him 2 screens away before engaging the fight, the enemies still aggroed him... if the priest hadnt gotten to him fast enough with withdraw that would habe been the end (ironman run))
starting POTD is ridiculous since the game tends to throw random encounters at you (act 1) that are harder than bossfights (at that time)... go ahead, play with 3/4 characters and try to clear the area south of gilded vale. Go ahead, the dungeon in gilded vale is harder than anything else you can face in act 1 - even most of act 2 isnt as hard.
POTD isnt as much about game mechanics as its about items and encounter knowledge.
As long as you leave yourself a way out in all fights or play a defensive party, you can start POTD quite early
If you are good at BG like games... start hard... (or POTD and reload in act 1 like mad -.-)
Things that make the game much easier:
Use a priest (not as MC, but have one in party), he can counter most CC abilities (sadly, you need quite some levels to do it)
Use withdraw - its the best spell to safe someone.
If you play monk - make him your DD, use quite some int (at least 15 - i prefer 18-20) and learn your positioning for torments reach (you can use less dex - the game isnt about min maxing if you just start), if you position well he will deal enough damage to get through all non bossfights easily.
Get a character with enough mechanic and go get the shade figurine and beetle figurine (you can get both early in act 2), they can change a lost fight.
Abuse doors! Every door with enemies behind it is a free win - really, its as easy as it sounds - even a 6 melee party can use a door.
Abuse walls and "end of maps", the game loves sending endless amounts of enemies, dont fight them in an open area, if there is a wall, perfect - only 3 sides left, so you cant get surrounded.
Look who's your MVP character in every fight, make sure he is in a good position in fights and get him the support he needs (sometimes the MVP is the tank - if he keeps aggro because of good positioning than nothing is worse than the tank leaving the position - so make sure he doesnt) - a monk with torments reach can hit 1, 2, 3 or every enemy with every use (if there are many enemies its the bug that makes it strong, however 4m reach ability isn't bad, and thats what you can get) so if your monk is your MVP... position him well!
The pathing is bad, you can abuse the enemy pathing (sometimes the pathing sucks enough that 1 enemy blocks 4 others... despite them having a way to your character) however make sure your characters dont stuck because of the pathing - its annoying and can cost you.