so with elder dragon effect, i only need to assign one for lethal, so if can i target soraka after assigning 1 dmg to all the other units on battlefield

Yes — in fact that order is mandatory, since Soraka has "I must be assigned combat damage last," which works exactly like Backline (Rule 826): you must assign lethal damage to every other unit at that battlefield before any damage can be assigned to her.

With Elder Dragon out, 1 damage counts as lethal for your damage, so you assign 1 to each other defending unit, then 1 (or more, if you have Might left over) to Soraka — her 4 Might doesn't require 4 damage. Small terminology note: combat damage assignment isn't "targeting," so nothing about her causing a targeting conflict.

One thing to watch if you're trying to wipe the board with her: all assigned damage is dealt simultaneously (Rule 467.2.c.1.a), and Soraka can still apply her replacement effect to units that die at the same time she does (Rule 370.4), so her controller can heal/exhaust/recall other units there with less Might than her — assigning her damage first or last in the order doesn't prevent that.