Update on the session's progress:
To sum it up, the barbarian and the rest of his new party went back to the Big Bad's fortress and managed to infiltrate it. For the first time I saw my players just sit down and plan out what to do next in-depth. I'll be honest, I could not have been more proud of my players...it was the first time this crew in particular planned things instead of rushing in blindly, though I did make it clear that they were infiltrating a base full of enemies and the boss was in "control" of two dragons. It makes sense that they were worried about (one of them was terrified and all jumpy, he couldn't stay sitting).
They managed to sneak up to the Big Bad's room and the boss battle began. This was the boss battle against the Cleric of Death with a grudge against the barbarian, the same fight that knocked out the barbarian for the first time in the campaign and the same fight the barbarian accused me of trying to kill him. In this fight they pretty much burned most of their novas.
After that, they snuck back down to free the captured villagers and had another battle with the cell guards and one of the army's captains. This was a medium encounter and since after the boss battle they used up all their healing items, they were left without any healing (aside from a 1st level cure wounds the druid was keeping as an emergency). They took moderate damage and used up more abilities.
Finally they led the villagers out as a big battle ensued and managed to escape into the mountain's cave system and close the entrance to delay the orc army. They were then tasked to scout the cave ahead of the large group of villagers to clear any threats. They estimated that they had around one hour, maybe two before the orcs caught up. The party opted to have a short rest (I DM the short rests to be 30 minutes) which allowed the monk, fighter and druid to recover their abilities and got some HP back.
Then came the big troll battle, as gory as it was hilarious. Along the way they found a troll feasting on a poor orc patrol. Battle ensued and they failed their knowledge rolls horribly, the best they could get out of it was that it was indeed a troll and it healed super fast. The players knew they needed fire or acid, but they know how much I hate metagaming and since the druid was out of spells, there was no way of "accidentally" using fire to find out. The group spent around 14 rounds hacking the troll apart. When they noticed the troll would just not stay dead, they started planning to hack it to pieces but even then they were having trouble doing it properly. The barbarian used his last two rages, the monk used all his ki, the druid his beast forms and the fighter his action surge and second wind eventhough they were unable to drop him even further down in HP. Finally the tiefling monk remembered that she had Hellish Rebuke and after the troll KOed the fighter and monk, it got burned down by hellish rebuke. End of the fight, they burned all their novas again and were horrified that there was still at least one more encounter to go. The troll was an easy encounter CR wise, but the lack of knowledge and fire made it harder (though not lethal by any means).
Standing by the exit of the cave was an orc group, in charge of guarding the area. With half the party at 1 or 2 HP, the barbarian out of rage and the druid out of both spells and beast forms, they were positively stressed. The barbarian was quite irked by the fact that I was making them fight again in those conditions while the rest seemed incredibly nervous. Again, the were forced to plan things out carefully, which again...I enjoy seeing. The orc party was a simple medium difficulty encounter, yet they stressed about it much more than they did the fight against the Boss, which they destroyed. Everyone but the druid was knocked out at least once, some even twice, but luckily the druid saved his cure wounds and had the Healer feat, allowing him to get the rest of the party up every time. In the end, they managed to do it.
End of the day, they had 1 Deadly Encounter and 3 Medium-Hard encounters with one short rest, due to it being a race against time. Everyone, even the barbarian, seemed quite pleased with their achievement, likely because it being so stressing made the victory much sweeter (Dark Souls style I guess). It helped that they saved around 200 people's lives, were considered heroes and handsomely rewarded with money and a keep.
Thank you guys for all the advice, it really and genuinely helped.