When I use Skuntank V's "Pursuit Blast" attack, do I get to do 120 damage if the opponent's Benched Pokemon I choose was switched out of the Active Spot or no?
No, Pursuit Blast only does 120 damage if the chosen Pokemon *retreated* during the opponent's last turn.
Source: Silver Tempest FAQ; TPCi Rules Team (2022-11-10)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH12_108
SWSH12_108