If I recall correctly, neither Merry nor Eowyn could have stood against the Witch King alone; Eowyn's attack gave Merry the opportunity to stab the Witch King, which he probably could not have otherwise done; and in doing so, he saved her life.
*baby Elf thinks Dwarves and Elves should unite to give the Telmarines what's good for them*
*baby Elf thinks Dwarves and Elves should unite to give the Telmarines what's good for them*