My brother and I played Tsavo highway on Legendary until the first checkpoint but then we had to get off. We recently went back on and continued from the checkpoint but when we finished the level we did not get the achievement. Is it because we got off and then continued from the checkpoint?
Yup, if it is co-op, you gotta start from the beginning. 
Okay, thank you. I was worried that might happen. We did it again, much faster than our first time. Onward, this is my first time playing the campaign. I was nevet allowed to get my own Halo game until number 4 and my first run through of that was on Legendary (on co-op) and now I wont let myself beat any Halo campaign any lower.
Yeah, I started on Halo 3 ODST (how weird is that?), but then went for Halo 3 Legendary as well.
Only in Halo Reach did I see myself forced to lower the difficulty at my first playthrough because those Elites were just impossible.
I beat them all Solo on Legendary eventually.
I went for Halo 4 Legendary on release night and it was awesome but I found that it was considerably easier to beat than the other games.
But that might have to do with the fact that I became better at the game after so many Legendary campaigns. 
Probably mostly because you got better. It might’ve just been easier too but I’m not sure. I used to think Zelda games were really hard but after about 7 years they’re still challenging but a lot simpler than they were then.