People saying "roll better characters" or git gud aren't getting it. The developers either didn't understand the CR (challenge rating) system, they didn't care, or they intentionally made this game balls hard.
I'm at the Sycamore dungeon at level 2. There are kobolds and mites with 3-4 levels in ranger, rogue, cleric, fighter, etc. all over this dungeon, in groups of 6-8. The party isn't min/maxed but the enemies sure are. Kobolds with sneak attacks, +4 favored enemy human, clerics who spam Hold person and channel negative energy, etc. DMs really aren't supposed to do this. Character levels are supposed to be somewhat rare, if every goblin has levels of fighter or ranger you are abusing the system.
Too be clear, this isn't a factor of tactics, it's just math. You might think a level 2 party can handle CR 2 encounters, but that's an
even match. Meaning a win should consume at least 50% of your HP and spells, and has a good chance of dropping 1 or 2 party members. For a 4 person party, a normal fight is supposed to be a CR-1 or CR-2 fight, weaker than the PCs. A CR equivalent fight poses a real threat, a CR+1 fight is a significant challenge. CR+2 or more is supposed to be a boss fight only, something the party would need to be in top shape to handle, with prep time and buff spells, just to hope to win.
Guess what? The CR of the fights in the Sycamore dungeon are all 4-6. For a level 2 party. So every battle is a boss fight. That's nuts. If I ran a game this way, my players would spit in my Mountain Dew. Or cut my brakes. This is the "normal" difficulty level too.
D&D characters are pretty fragile from levels 1-3, you really have to go easy on them.
*edit*
Cael has corrected my explanation of the CR system below, it's not quite as bad as I originally made it out, but the fights are still way out-of-depth.