Yes, I think it is, if the snipers were properly trained and coordinated their effort. I think the timing is important.
Unfortunately, almost everything we know about Time Lord mortality comes directly or indirectly from The Doctor (e.g. some of it via River Song). And, to my knowledge, we've never seen a Time Lord die permanently on-screen.
However, we do know that it's possible to kill them, and have some information to go by (depending on how much you're willing to believe The Doctor):
- Daleks, obviously, killed millions of them in the Time War.
- House killed hundreds of them on his pocket-universe planet.
- River certainly believed that she could kill The Doctor in "The Imposssible Astronaut"
- The Eighth Doctor would have died in a spaceship crash, without the Sisterhood of Karn's intervention.
- The Eleventh Doctor would have died when River Song poisoned him with Judas Tree extract.
Daleks obviously vaporize their target instantly, so I presume that would be the end of it. As they've largely evolved to kill Time Lords, I think we can take for granted that they know the best way to do it.
In the cases where physical/chemical damage alone has to suffice, I think the central theme behind each of these examples is that something happened to the Time Lord that interfered with the regeneration. This lines up with something The Doctor has said a few times: if you kill him while regenerating, or if you injure him enough that he dies before regeneration can happen, he will die. The events of "Lets Kill Hitler" seem to confirm this, since the Judas tree poison specifically blocks the regeneration process, and thus, is able to kill The Doctor. So, in order to kill a Time Lord, you just need to do something to make sure he can't regenerate.
For example: if House killed a Time Lord, then immediately decapitated him and removed his hearts, the Time Lord's body would die very quickly. It's possible that he wouldn't have time for regeneration to repair the body. Similarly, the way River was instructed to kill The Doctor was to kill him, then shoot him again when he began regenerating. This was supposedly enough to halt the regeneration process and kill him permanently.
In the mini-episode "Night of The Doctor", the Eight Doctor really does die from the spacecraft crashing full-speed onto Karn. He's only revived because the Sisterhood found him and healed him long enough for them to trigger his regeneration -- it didn't even trigger by itself, because he had died for good.
Given all of that, yes, I think 8 snipers is probably enough to kill a Time Lord for good, but I think they'd have to be very careful how they did it. I don't think just shooting 8 bullets into Missy at once would work unless they were very lucky and completely shredded her internal organs. However, I think if they staggered their shots:
- Half of them shoot her hearts/brain stem
- Wait a moment to see if regeneration begins
- The other half shoot again to stop the regeneration
I think, based on what we've seen, it would probably work.