This app was mentioned in 6 comments, with an average of 1.33 upvotes
https://play.google.com/store/apps/details?id=com.christian.bar.dndice
It allows creating presets.
To your situation there are two approaches. One is simply create a 1d20 + 2d6 (or whatever your damage is) preset and every time you touch the preset name it will throw these dice. This is the one I recommend for you since we're talking about 6+ attacks and the virtual space has only so much "space".
One other approach the app supports is actually create a preset with for example 3d20 and 6d6 but color code the dice in a way that 1d20 and 2d6 are yellow, 1d20 and 2d6 are green and so forth. That way one single tap on the preset is all you need. Of course this will be a lot of dice on the screen at the same time.
For anyone looking for a replacement of Prime Dice. I found DnDice and it seems to do the job nicely. Like I said before you can still use the free version of Prime Dice.
https://play.google.com/store/apps/details?id=com.christian.bar.dndice
Don't worry - I use those programs too when I forget (aka am too lazy) to bring any 'real' dice with me.....
DnDice is free(!) and seems at least as good as any of the paid ones:
https://play.google.com/store/apps/details?id=com.christian.bar.dndice&hl=en
You could use a virtual dice roller until you get a set or 30.
Android: https://play.google.com/store/apps/details?id=com.christian.bar.dndice&hl=en
IPhone: https://itunes.apple.com/us/app/dice-dragons-rpg-dice-roller/id581040723
I like DnDice; used it for years, highly customizable, no big ads but offers additional features for purchase.