If an opponent controls an Elder Dragon, and I control a unit which has more might than the elder: Then, I play Rampage, which deals damage to each other's units. Then does my unit die to elder because it counts as damage done by the elder, or does it survive because it doesn't count because it counts as damage done my my spell?

Your unit dies. Rampage has the two units deal damage to each other, and Rule 417.6.b.3 makes the unit itself the source of that damage — so the hit your unit takes is damage marked by your opponent, i.e. "their damage," not yours. Elder Dragon's passive alters lethal damage for exactly that damage (Rule 142.4.c), so any non-zero amount is enough to kill an enemy unit regardless of Might; your unit having more Might than the Dragon is irrelevant. Counter Strike or another prevent effect would be the only thing that saves it, since prevented damage is never dealt.

Note the reverse direction still works in your favor: the damage your unit deals to Elder Dragon is your damage, so the passive doesn't protect it — if your unit's Might exceeds 10, Elder Dragon dies too (both die in the same cleanup).