Meriadoc the Hobbit at the Pelennor Fields (LotR V.6)

Tolkien makes a point of making Merry distinctly unheroic in this scene.  He starts out crawling on all fours like a dazed beast, blind and sick (C.5). He can't even look up (C.6). 

Amazement conquers his fear, briefly, when Dernhelm boldly laughs and reveals herself (C.13). Then pity and great wonder suddenly awakens "the slow-kindled courage of his race" (C.14).

That courage forms itself into a truly heroic resolution, "She should not die, so fair, so desperate!" (id.).  But it is promptly deflated to the more realistic "At least she should not die alone, unaided" (id.) – presumably because his hobbit-sense tells him the help of a dazed and untrained halfling is not likely to save her from death.

Even now, having resolved (essentially) that he should die with her in an almost certainly futile attempt to aid her, he "hardly dared to move, dreading lest the deadly eyes should fall on him" (C.15).  It'll be hard to aid her at all unless he moves, of course.  But if I've got this straight, he's afraid of being looked at by the Ringwraith. Seriously, it's not as if the Ringwraith is shooting lasers from his eyes or anything.  Yeah, yeah, it's a grave spiritual peril, I know – and Éowyn quails from his gaze as well – but still, Merry's reaction does not exactly inspire confidence in his martial prowess.  It's not too surprising that the Ringwraith heeds him "no more than a worm in the mud" (id.).  Merry is obviously an everyman, a negligible quantity.

Éowyn and the Ringwraith exchange blows, or rather, she kills his steed and he knocks her down to her knees and prepares to finish her off, when Merry intervenes.  This is his great battle-moment.

And what does he do?  He stabs the Ringwraith's leg from behind and below, while Éowyn unwittingly distracts him.  At most, the blow could be crippling rather than fatal.

So it's two-on-one, stab-from-behind, at the back of the Ringwraith's knee; not very heroic, but the best he can do.  And it is enough.

Huzzah for the everyman?  Not so fast: critically, he had collected a dagger en route that turned out to be magic.  "No other blade, not though mightier hands had wielded it, would have dealt that foe a wound so bitter, cleaving the undead flesh, breaking the spell that knit his unseen sinews to his will" (E.2).  What should we make of this chance, if chance we call it?  As so often happens in LotR, a sort of providence may help those who at least try to do the right thing, even when they are certain their effort is hopeless and doomed to failure.  The sword required a hand to wield it, and it turned out to be Merry's.  But there was no guarantee, a priori, that his sword would be a magic one "wrought [...] slowly long ago" by foes of the sorcerer king (id.).  On the spur of the moment, with no assurance of success, he had to crawl in the mud under the deadly eyes of the Ringwraith and strike his feeble blow with an unnamed blade randomly dealt out to him, without special ceremony, by a singing stranger.

Here is "a sudden and miraculous grace: never to be counted on to recur" – but Merry had to do his part, in whatever way he could manage.