I attack with an 8 might creature at a BF with 8 creatures. I have an Elder Dragon and their creatures have Tank. Do I need to kill 1 the Tank or does the lethal from Elder Dragon make me select each one, just choosing 1 the Tank?

Yes, you can assign just 1 damage to each of the 8 units, including the Tanks, and kill them all.

Here's the breakdown:

Elder Dragon makes 1 damage count as "lethal" for any damage you deal (FAQ #8762, #10411). So for damage assignment purposes, each unit needs only 1 damage assigned to satisfy the lethal requirement.

Tank says all units with Tank must receive lethal damage before you can assign to non-Tank units (Rule 815.1.c.2). Since all 8 enemy units have Tank, there are no non-Tank units to worry about — and per Rule 815.1.c.2, when multiple Tanks are present, you can assign to them in any order.

So with 8 total Might: assign 1 damage to the first Tank (lethal ✔), then 1 to the next, and so on. All 8 get exactly 1 damage. Since 1 damage is lethal thanks to Elder Dragon, every one of them dies in the following cleanup.