You will learn to appreciate Marcus, especially if you wait until around chapter 20 to use him, since that's when everybody should start promoting. But still, he's a magnificent utility unit with great weapon ranks, solid stats, the weapon triangle on his side and has the mounted movement bonus making him an omnipresent threat to all enemies.
The gaiden missions aren't hard to get in 7, unlike some of the missions in 6. Unless you're going for 19xx, but I assume you're not playing in Hector mode so that's moot.
Easiest: FE8 (Sacred Stones) for the GBA. It's so easy that promoted enemy units have the stats of the same level UNPROMOTED enemy units. So you'll be running into Druids with stats of Shamans. Also, if you get the Japanese version, the rookie units have higher growth rates than the US version, even though they get 50 levels (10 for their rookie class, 20 for base and 20 for promoted). That was their attempt to balance them out... even though they never bothered to make promoted enemy units, you know, have better stats.
Hardest: That's up for debate, really. Though a lot of people say FE5, AKA Thracia 776 for the Super Famicom (SNES). I personally haven't played it beyond one mission, so I can't say for certain if it is the hardest.