So how does that differ from being beaten up? Is that not considered "serious harm?"
Not being an attorney, I'll have to take your word on the case law, but I fail to see how someone attacking me solely because he wants to beat me up doesn't in itself constitute "the commission of a forcible felony."
Which you'll note in (1) doesn't have the unlesses attached that (2) does.
The test the jury applies, using their sense of "reasonable," is that in order to use deadly force, a person must reasonably fear serious injury or death. There is no bright line, but a shoving match, slap boxing, maybe even punches to the gut would not meet that threshold.
But, if a victim of a beating is rendered helpless, and the beating goes on, it's likely a jury will find that the person being beating reasonably fears serious injury or death. The test isn't extent of injury, it's "put yourself in that predicament, and consider what downside you would fear occurring."
Here is a case that attempts to clarify when 776.041(1) applies.
Giles v. State, 831 So. 2d 1263 (Fla. 4th DCA 2002)
The instruction is normally given in situations where the accused is charged with at least two criminal acts, the act for which the accused is claiming self-defense and a separate forcible felony. See Marshall v. State, 604 So. 2d 799 (Fla. 1992)(holding that section 776.041 jury instruction was proper on claim of self-defense to charge of felony murder where underlying felonies were burglary and aggravated battery); cf. Perkins, 576 So. 2d 1310 at 1311. Here, by contrast, Giles committed [*1266] only one act, the alleged aggravated battery.