Cure4 should be healing for more HP. It always healed for about 1500-2000 HP when I was playing the SNES version (both FF"II" US and FFIV Hard Type). Equip Rosa with a White Robe or whatever it's called (it's an armor found in the final dungeon, and it enhances Rosa's white magic a lot).
That sounds like a clever strategy, although it won't work for long. Zeromus will frequently cast Black Hole, which will Dispel all the positive status ailments your party has, so your characters will lose the Reflect status when Zeromus casts Black Hole.
If you want some more advice, I think the best advice is to lower the battle speed. With the fastest battle speed, Zeromus can cast 1-2 Big Bangs in a time when Rosa can cast only one Cure4. With the slowest battle speed, it's the other way around, so you'll have more time to recover from Zeromus' Big Bang. Have Kain Jump, Rosa use Cure4, Cecil use physical attacks and maybe heal if needed, Rydia cast Ashura or Bahamut (although Zeromus will counter Bahamut with Flare or Bio) and Edge throw all the extra weapons you have.