If I have Rengar Trophy Hunter with a buff (7 might) does he die when targeted by Singularity followed by a Stupify?

Yes — Rengar dies.

With the buff he's at 7 Might. Singularity marks 6 damage on him (6 < 7, so he survives that alone), but when Stupefy resolves afterward it reduces his Might to 6 while the 6 damage is still marked. Lethal damage is checked whenever a chain item resolves: 6 damage ≥ 6 Might, so he's killed (FAQ #4751). The order doesn't matter — even if Stupefy resolves first, Singularity's 6 damage still makes the kill. If Stupefy never resolves, he'd survive at 7 Might with 6 damage marked.