Hey Crona, I've been told I have to rate this so here I am :) This is actually a really strong team, but it does have a couple of issues. Like you said, Sun teams and Scarf Terrakion can cause you problems, but in addition Gastrodon (who literally walls your entire team) and some offensive Water types such as Scarfed Keldeo are also threats (there's also some random stuff such as DDMoxie Salamence which can practically sweep you clean if it gets a boost - so just be careful not to lock Genesect into Bug Buzz if you see one).
Okay, so the most significant change I think you should make is to run a Focus Sash Dugtrio instead of your Landorus. This gives you a much, much easier time against all weathers, but especially Sun; in addition, it traps most of the things which counter Tornadus and Genesect, such as Heatran and SpDef Jirachi. Having Forretress makes it really easy to maintain the Focus Sash; finally, Dugtrio traps a lot of the common Stealth Rock users, so if you play right you often won't need to Spin at all.
Something else you might like to try, although this is much more of a preference thing than a clear improvement, would be to run a mixed defenses Amoonguss over Lanturn and then some form of Jirachi (SubCM is the most popular for this type of team, most other movesets would be fine so long as they have enough bulk to check Tornadus) in place of Genesect. This change doesn't hurt you defensively at all, but helps a lot against Gastrodon and most Fighting types (Breloom, Keldeo, Terrakion, Conkeldurr), as well as giving you some nice overall extra bulk.
Hope you like my suggestions! Again, nice rain team, and good luck :)