If I use Charizard's "Roaring Resolve" Ability with only 10hp left, does he get KO'd when I place the 2 damage counters before or after I search & attach 2 {R} Energy cards?
You must finish all the text of the Ability before checking for knockouts, so yes you would search your deck and attach {R} Energy cards before KO'ing Charizard.
Source: Team Up FAQ (2019-01-18), TPCi Rules Team (2019-01-31)

Relevant cards

Note: All applicable cards may not be listed below.
SM9_14
SM9_14