It's possible, yes, but really, really hard without Neptune or Shiva. The problem is Neptune in Oceanus Falls. You could always farm yourself a Hera and Ultra Gravity him to death (which also takes forever), but the problem is that you'll still have to deal with a Neptune in Hera Descended.
My suggestion is that, if you were to only have one REM monster, it should be Neptune. Hydra's Poison is just way too useful, and it becomes apparent if you don't have it (it was NOT fun, hitting Neptune for 1 over and over, with Lilith's poison only hitting for like 300).