Hey Jr, looks like a really solid team. Nice to see an RMT without Genesect around, and that Scarf Jellicent set is absolutely brilliant!
So, on to the rate. I've never really found Swords Dance on Breloom to be that great, to be honest. You only get to set up late game, and with 3 Choice users, SD seems like it would kill your momentum. Like you suggested, I would recommend putting Low Sweep over Swords Dance, which gives you a more powerful Fighting move while allowing you to stay in and destroy Starmie and Keldeo. I think you need more Spe EVs to outspeed Starmie even after the reduction, however. You may want to switch to Life Orb if you opt for Low Sweep as well.
I might also put Body Slam over Thunder on Jirachi. Although this does leave you more open to bulky waters, it's not like Jirachi is going to 2HKO Jellicent or Vaporeon anytime soon. This allows you to run a Calm Nature (+SpDef, -Att), and you'll never know when the extra speed will come in handy. This also gives you the ability to paralyze ground types, which is nice, because the less Earth Powers that Landorus is firing off at you, the better.
These are all minor suggestions, congrats on Top 20, and once again awesome Jellicent set! Luvdisc'd
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