Elche set up a promotion play-off final against Granada after beating Real Valladolid in the last four on Sunday.
Elche lost the first leg 1-0 and their task became even harder when Oscar doubled Valladolid's aggregate lead after 23 minutes.
But the match changed just before half-time when Edu Albacar netted twice in two minutes to put the hosts 2-1 up on the night.
Angel Rodriguez then grabbed Elche's third 12 minutes into the second half to put them 3-2 up overall.