There are some people that disagree on some things in gaining proficiencies... so I tested this out for myself:
Tested @ level 21 in Arena ONLY... not welling or Catacombs
9669 (staff)
1st attack (no gain)
2nd attack 9670
3rd attack 9671
4th attack (
killing blow) 9672
That is 1 point per hit... and 1 point on killing blow
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Based on this...
1. You do not gain Prof points on first attack
2. You can gain consequetive points on each attack
3. You can gain point on Killing Blow
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If you disagree with this, then test it yourself to find out.
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2nd Test
9672
9672
9673
9673
9673
9673
9674
9674
Low damage compared to previous monster... same level same item
Looks Like it can be random
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3rd TEST... this changes something... same level/same item
Started: 9674
1st hit........................
9675 
Yes I gained a prof on first hit
2nd 9675
3rd 9676
4th 9677 (killing blow)
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4th Test... different again
9677 (1st hit)
9677 (2nd hit)
9677 (killing blow)
No points
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5th Test... same as 4th test... 3 hit kill and no prof gain
Based on Wigwams post... going to log back on to see if I have gained some profs based on Server side issues...
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Okay... this makes no SENSE now... I just closed down and logged back on...
My Staff Prof reads: .... ....
9674 (that is a loss... as if I did not kill those monsters)
Could be a glitch... and it is annoying at the least...
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just noticed that in TEST 4 & 5... I had gained a level... so of course I get no Profs...
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4 attacks.... gained 3 prof points (not for killing blow)... so back to 9677
5 attacks... gained 4 prof points (1 for killing blow and 1st attack)... 9681
4 attacks... gained 4 prof points (1 per attack)... 9785
4 attacks... no gain...
7 attacks... gain 3 prof points
5 attacks... gain 3 prof points (killing blow also)
7 attacks... gain 2 prof... 1st hit and killing blow
This post was edited by izParagonzi on Jan 16 2010 04:32pm