Yeah, I think you're doing something wrong there. For starters, a Druid at 2nd level would have around 17 hit points (8+5+4 for a 14 CON). He turns into a Brown Bear with 34 HP. A brown bear has an 11 AC, so the Hill Giant will hit it on a 3+. This is an 85% to hit, and the Druid itself likely has only around a 13 or 14 armor class, so after transforming the Hill Giant will hit 75% of the time. So the druid changes, the Hill giant deals an average of 30 damage. This will bring the Druid down to 4 hit points in his bear form. On the next round, the druid will die, because the Hill giant will deal another 28 damage on average. The first 4 will be to the bear form, and the last 24 will be to the druid himself, killing him instantly. Either you're underestimating the amount of help the wizard did, not taking into account that a Hill Giant with 70 hp would have far less experience, or something with the math was wrong at some point.
I don't remember the exact combat sequence and I don't have my books with me but I don't think any serious rules errors were made. The fight went something like this:
1.Party surprises giant and wins initiative, getting two rounds worth of attacks in.
2.Giant hits once and misses once. (lucky druid)
3.Another round of attacks on the giant, druid gains hp from combat wildshape.
4.Giant hits once, turns druid back to human form, misses (druid has 16 or 17 AC, i think, due to shield studded leather and dex)
5.Druid morphs back into bear (bonus action) so both characters can still make a round of attacks on the giant.
6.Giant hits druid twice, changing him back to human form and then severely injuring him.
7.Druid hits giant with Shillelagh, wizard casts sleep and rolls 20+, putting the giant to sleep.
The party got very lucky, but I think it was pretty legit. They did get a nice crit against the giant at one point. But I still rolled all the dice in front of the players, tracked human and bear HP separately, and no one had any magic items or even feats.
It was definitely a monster I expected the players to entangle and run from, not turn to fight. The wizard was useful, but the druid's ability to absorb massive damage was clearly the party's saving grace. At 3rd and 4th level, the druid has still been good, but his power seems more appropriate for these levels.