Rek'Sai has 4 might and I have two 1 might units, but I used Wages of Pain, so does 1 of my units live and Rek'Sai die?

No, both of your 1‑Might units die, and Rek'Sai dies too.

Here's the combat breakdown after Wages of Pain deals 3 damage to Rek'Sai:

• Rek'Sai has 4 Might with 3 damage already marked.
• Your two 1‑Might units attack (total 2 attacking Might). Rek'Sai defends (total 4 defending Might).

Attacker (you) assigns your 2 damage: Rek'Sai needs only 1 more damage to reach lethal (3 existing + 1 = 4 = its Might), so your 2 damage pushes it to 5 marked damage — Rek'Sai is killed.

Defender (opponent) assigns Rek'Sai's 4 damage among your two units. Each 1‑Might unit needs 1 damage for lethal. The opponent assigns at least 1 damage to each — both take lethal damage and are killed.

All damage is dealt simultaneously, then any surviving units would be healed in Combat Cleanup — but there are none. All three units die.

(Rules 460.2.c.3–4: lethal assignment priority; Rule 143.2.a: a unit is killed when nonzero marked damage equals or exceeds its Might.)