Ambipom: On the Fake Out, if it has a Life Orb, I automatically use Rain Dance. If it does not, I use either Rain Dance or Waterfall depending on whether I think it's going to taunt me or not. Or I'll just use Explosion if I don't feel like dealing with it.
Uxie: I usually start by Spiking against Uxie so that if it's TrickScarf I can get 2-3 layers down immediately. If it's not, Qwilfish can low-investment Psychic/Zen Headbutt, and is faster than most Uxie variants, letting me get up a layer of spikes and rain.
Froslass: Waterfall straight out. If it taunts me, I win. If it spikes, I use RD on the Destiny Bond or second Spikes, and then kill it. If it has tbolt, no matter, Qwilfish can survive...and if it Tbolts me, it's risking me getting rain up.
Regirock: Rain Dance, Spikes, Spikes, Explosion/Waterfall if I'm still alive. I win pretty handily.
Omastar: Spikes, Spikes, Rain Dance, switch to Ludicolo. The two layers turn most of rain's checks into 2hkos (except water absorbers obviously).
Moltres: Qwilfish survives all attacks from all variants of Moltres barring random shit like LO HP Ground. As long as it doesn't flinch me with Air Slash, I get rain up and kill it.
Swellow: See Moltres.
Now, one pattern you'll see here is that Qwilfish works very similarly to Electrode as far as its matchups go (except for doing much worse against Omastar and much better against Regirock). However, the reason I use Qwilfish over Electrode is because it's not nearly as vunerable to common Scarfers in the lead position (Moltres, Primeape, etc), because it can set up hazards, and because it outruns base 80+ scarfers in the rain (important in case Lead ScarfVenu is stupid enough to use EQ, which doesn't ohko, or if Sleep Powder misses).