Dies to Removal

Rending Volley [mana]R[/mana]
Instant
Uncommon
Rending Volley can't be countered by spells or abilities.
Rending Volley deals 4 damage to target white or blue creature.




http://www.mtgfrance.com/download/file.php?id=5916&t=1

Still not as good as Combust, but the cost reduction is sweet.

Technically not as good as Combust, but functionally probably better. Trading can't be prevented and 1 damage for 1 mana is probably fine considering the stuff you want to hit in Modern is usually a 4-butt. Pestermite, Deceiver Exarch, Celestial Colonnade, Restoration Angel, Snapcaster. The cost reduction seems important against those decks.

so saucy in modern

I'm trying to think about what Modern creatures you SB combust in for that also have 5 or more toughness, but most of them don't see play (Baneslayer Angel) or Combust isn't really the ideal sideboard choice against that deck (Siegefather).
1cc "fuck a Splinter Twin" outside of Abrupt Decay is also leagues better than 2cc for the same effect. The -1 damage and lack of prevention negating is more than accounted for. I think this is a hit.

Combust is sometimes relevant against Siege Rhino, but that's pretty narrow. 1 CMC is a big deal. This card is $cashmoneycraycray$

Roast and Dismember are probably better if you're planning to fight rhinos with red, because those spells are applicable against the spectrum of threats rhino decks will throw at you vs. 1 creature that is good.