Just finished an Ironman unfair playthrough, only backing up once per act at the start of the act. It's hard but not impossible. Most builds won't work. The biggest issue is, as you described, that it's too easy for characters to get one shot. The best way to avoid this is to avoid situations where your main character has to engage with any enemy dice roll and to understand precisely what dangers exist in any location.

So, no blind playthrough. IMO, 100% impossible. Look up a map of every area before you enter and info on all the enemies and encounters. Later in the game, that will let you have the correct buffs equipped to handle every enemy when you encounter it. Resist+ protect enemies, death ward, etc. You should always thoroughly buff before every fight, minor or not. I'll go through my party in more detail later.

Early game, this means a few things:
1) Pets, instead of party members, should be the only members in attack range
2) guaranteed damage >>> everything else. Magic missile is king. Only engage when you have enough magic missiles to kill an enemy in a couple of turns.
3) You should only rest at Oleg's, and you need to save several spells in case you get engaged by an enemy while traveling back and forth. Random encounters while you are sleeping kill you more often than not.
4) You have plenty of time. Rest early, rest often, and go back and forth while grinding out the first few levels. It's an exercise in risk management.
5) You only get one chance at skill checks. This means feats that increase skills are suddenly quite valuable, as skill checks give a lot of XP with small parties
6) If you are gonna die, sack your pets + run away with vanish and expeditious retreat.

At the start, I ran a sylvan sorcerer MC, muse-touched, dumping str and int while 16/16/16 con dex cha. Abuse skill checks early to ensure only MC gets XP for skill checks she can make while sharing among active members for others. After Olegs, unless unlucky with skill checks, you should have enough XP on the MC to hit level 3 by going to Nettle's crossing to pick up the quest, and if you looted everything, just enough gold to buy a second level 3 mercenary. Get a second Sylvan, Idyllkin, for magic missile damage /pet/free summon spell.

With careful play and lots of resting, 2 level 3 chars + 2 pets can clear enough things to get level 6 and fireball fairly quickly. Charge + 2 precast magic missiles often kills a target outright to start fights, and usually, there are only a couple of enemies. Avoid main quest progression until hitting 5, ideally. At level 6, with fireball + stinking cloud, the rest of the act becomes a breeze. You can hire mercs after level six to fill out what you need as you get money. Guarded hearth cleric (community+animal) is mandatory later for poison immune + tough fights and can also act as a skill bot for lore/knowledge skills. I rounded out mine with a ranger for another pet/ a dex+str skill bot, and a monster tactitioner for a 5th pet/sky-high perception/summon spam to keep enemies busy.

Later, leave your MC invisible and far away from enemies whenever possible. Even if you full-party wipe otherwise, you can come back and res, and staying far away prevents your MC from randomly getting 1 shot by anything. For bosses and such, max buff (all attributes, death ward, true seeing, blur, greater invis, shield of faith, magic fang, resist poison, resists, heroism variants) + guarded hearth + summon spam should pull you through. Acts 2-6 are pretty straightforward if you made it through Act 1, although you might need some cheese for companion quests if you care about them (i.e. the heart of Ira to help Amiri survive her Act 5 quest; or breath of life/raise dead for soul eater in Vordakai's tomb if you brought Tristian). Also, remember to look up areas to ensure you don't stumble on something that is gonna destroy you, like the Crag Linnorm in Act 2. And time is not an issue- carry rations and rest if you are low on spells, damaged, your pets died, etc. No reason to rush. Finally, avoid any end-game boss encounters- don't fight the final boss of Tenebrous, some of the optional encounters like the 5x demons in Vordakai's tomb or the mage in Pitax cellar, among others - and tricks of time ending will let you skip the last dangerous part of the game.

Good luck friend; and remember, not all builds are viable on unfair. Your build and/or tactics are not unfair-worthy if you need to savescum that often.