I'm asking cause I just killed two giants and a mammoth at level 3/4(started fight at level 3, ended it at level 4) as an altmer with the atronach stone(lots of magicka, and with highborn the slower magicka regen doesn't matter). I played on adept, and had no followers or help from guards. I used no exploits or mods.

But I'm pretty sure there are people who've done it earlier, probably with sneak archery or as a warrior with vegetable soup.

Now that I'm done sneak bragging, it's your turn

Level 2. My very first game. I went form Helgen straight to Riften. This took me past the Orc settlement with the Giant outside the gate. I was used to Oblivion where everything is leveled so I eagerly attacked him with my sword. The just make him mad. He took off after me and I backpedaled, firing arrows into him.

We ran from the Orc Settlement over to Riften, then back in the other direction. I ran out of arrows and started to hit him with my sword and then run away from him while I healed. Then I'd turn and hit him again. Five or ten minutes later he finally bit the dust. "Wow, this isn't Oblivion," I thought to myself. "I'm going to have to be more careful about what I attack from now on."

From the very first level. I lured him into a group of guards. I know you may think that this does not count, but I worked really hard to get the funny guards attack the giant. One kept saying that his bet is on the bigger one. That's no way to treat a helpless elf .

I do not remember, but I try to avoid them most of the time. When I do feel like it, I go in for the stealth kill.