If I only have one energy card attached to Excadrill, can I use its "Omega Barrage" to attack with "Dredge" first to attach two energy cards, then attack a second time with "Mach Claw"?
Yes, that would work.
Source: Primal Clash FAQ (2015-02-05), TPCi Rules Team (2015-02-05)

Relevant cards

Note: All applicable cards may not be listed below.
XY5_121
XY5_26
XY5_69
XY5_81
XY5_97