There really is no reliable way to get 20+ DV
Vindicator Minigun (Base Damage 20 P Full Auto)
Dicepool of 15 ,Edge 6 = Damage of 30P is possible
HMG (Base Dam 16P for Full Auto ) same Dicepool & Edge = 25P possible
with a possible Dance
Medicineman
Unless I missed an errata, or my math is wrong (both are entirely possible), Vindicator Minigun Base Damage: 6 + full auto bonus: 9 = 15 Modified Base DV, which would still require a dice pool of 15+ to hit 20DV. If you need to roll at least 15 dice, and hope that you get that many hits, I don't know if I would call that reliable. (I know the minigun uses 15 rounds per full auto burst, but I can't find anywhere in the rules where it states that its damage is increased from that. The way I read it, you still only get +9DV, it just costs more ammo to get there).
So how does one reliably get over 20dmg then?
There really is no reliable way to get 20+ DV with a single attack.
Any weapon fired in full-auto with awesome dicepool(20+) does 20+ damage quite reliably.
With a dicepool 21 dice you get on avarage 7 hits combined with the +9DV for full auto gets us to DV of 16 meaning even a machine pistol loaded with standard rounds do 20 points of damage.
Get a bigger gun and better ammo and your doing 20+ damage quite reliably.
As above, I don't know if I could call needing to rely on rolling a 21+ dice pool reliable.
Yes, these are both possible, but in what extreme circumstances are you going to have 21+ dice pool with no situational modifiers? I know its do-able, but how overly specialized do you need to get? It's viable sure, but I guess I'm just not a big fan of overspecialization.
Then again, being overly specialized or not is not what this thread is about, so both of these options are totally 100% valid - assuming you build up your dice pool to meet the minimum required hit average.