How should Genesect be used in battle, and in what situations are it best in?
Genesect is, in my own opinion, the best U-turning Pokemon in the entire game, and arguably the best Choice Scarf user. U-turn is an amazing tool that lets a player dent, or even KO something like, say, Latios, and then give you what amounts to a free switch. Denting something like Kyoge is extremely important in that
it gives your team free momentum.
Picture the following situation:
It is turn
one, and you decide to use a Physically based, Choice Scarf Genesect while your opponent sends in a Kyogre. In addition to Genesect, you have a Lustrous Orb Palkia on your team. Now you can simply switch to Palkia to deal with Kyogre but why just switch when you can use U-turn? Needless to say, you use U-turn, hit Kyogre on it's weaker physical side, and make it lose 50% of health. Kyogre then fires off a Surf right into your Palkia. Palkia takes minimal damage. The turn ends.
It's now turn
two, and it's Palkia vs Kyogre. You know that Palkia will outspeed all non Scarf Kyogres. That means your opponent is ether going to switch, something that allows you to predict, or your opponent leaves Kyogre in and you kill it with Thunder.
Genesect has already given you a tactical advantage and it's less than four turns into the match. With it's powerful STAB U-turn, Genesect has helped you gain one heck of an early game advantage. In fairness, the Kyogre set could have something like Thunder Wave, which lets it mess with Palkia, but overall, you've dealt a lot of damage to the beast, and probably a KO/ high damage if Kyogre switches out.
Now this is not Genesect's only skill; with it's 99 base speed, Scarf Genesect is a fantastic revenge killer. With U-turn, it can KO many Psychic and Dark Pokemon, with Iron Head, it can smash the Kyurem Forms, with Ice Beam, it can murder Shaymin-S and, Rayquaza (even after a dragon dance), with Flamethrower it can melt Forretress, Ferrothorn, and more. It also has Explosion to literally go out with a bang, something that destroys things like Ho-Oh. Finally, it has Thunder/Thunderbolt in case of other Pokemon. Genesect's Download boost can potentially give all of the above options even more power. Genesect is a very powerful killing machine. With a scarf, it can kill and revenge kill many things throughout a match.
Overall, Scarf Genesect is great for momentum, and great for revenge kills. It can clean, scout early game, and more.
P.S. You can also use Shift Gear for a sweeping set, but I never see it. It's kind of bad. Extremespeed and Blaze Kick can be used too if you wish.
I hope this helps :)