If I use Hypno's "Pendulum Influence" attack to use Ampharos ex's "Thunderstrike Tail" attack, can I do the extra damage even if I don't have 2 Energy to discard?

Yes, Thunderstrike Tail is somewhat unique in that it has you choose whether or not you want to do the extra damage, and then you discard 2 Energy if you can. If you only have 1 Energy attached to Hypno and choose to do the extra damage, you would discard 1 Energy. Even if you were able to use Thunderstrike Tail with 0 Energy attached to your Pokemon, you could choose to do the extra damage.

Relevant cards

SV1_083

SVP_016