Counter Attack

Counter Attack is defensive skill available to players 400 and above.
Unlike other skills, putting the maximum skill points on this one is not necessarily good.
Here's the official description :

Uses 0.25% extra stamina (per point) to add 0.25% to both attack and damage. (Both values are rounded up, vs. creature only)

Without doubler, that simply means 2 stamina per round of attack. That means this buff is completely useless without a doubler. Since it also means it uses extra stamina, this buff is useless if you are already 1-hitting without the buff. It is only useful when the extra attack/damage allow you to 1-hit, when without the buff you would be 2-hitting. However, selecting the level of the buff you will want can be tricky.

3x doubler
With 3x doubler, up to a level 133 CA, 1 extra stam is used, making each round of attack using 4 stam instead of 3. With a level 165 Wither, you have almost a 1/3 chance of 1 hitting, so that makes it an average of 5 stam per attack (2 attacks will use 6 stam, while 1 attack will use 3 stam). This means this buff is still very useful if it can convert the 2-hit into a 1-hit. But the extra 33.25% attack/damage must allow you to 1-hit. Since the level 165 Wither is 5 stam per attack average, that makes level 134 or higher CA quite useless.

Level 1-133 CA : 4 stam / round of attack (Level 133 = 33.25% extra attack/damage)
Level 134-175 CA : 5 stam / round of attack (Level 175 = 43.75% extra attack/damage)

Compare ::
Level 165 Wither : 5.01 stam average / creature
Level 350 Wither : 3.9 stam average / creature

4x doubler
With 4x doubler, up to a level 100 CA, 1 extra stam is used. 5 stam per attack, compared to a level 165 Wither which will use 6.68 stam on average. Even using the level 350 Wither potion is still inferior because that averages 5.2 stam per creature. Above 100 CA, 2 extra stam means 6 stam per attack, still better than a level 165 Wither.

Level 1-100 CA : 5 stam / round of attack (Level 100 = 25% extra attack/damage)
Level 101-175 CA : 6 stam / round of attack (Level 175 = 43.75% extra attack/damage)

Compare ::
Level 165 Wither : 6.68 stam average / creature
Level 350 Wither : 5.2 stam average / creature

7x doubler
7x doubler comes from the Super Doubler Potion. These potions were dropped by creatures found in Scavenging (I cannot remember the name of the creature). These potions can be traded.

Level 1-57 CA : 8 stam / round of attack (Level 57 = 14.25% extra attack/damage)
Level 58-114 CA : 9 stam / round of attack (Level 114 = 28.50% extra attack/damage)
Level 115-171 CA : 10 stam / round of attack (Level 171 = 42.75% extra attack/damage)
Level 172-175 CA : 11 stam / round of attack (Level 175 = 43.75% extra attack/damage)

Compare ::
Level 165 Wither : 11.69 stam average / creature
Level 350 Wither : 9.1 stam average / creature

10x doubler
10x doubler comes from the Potion of Supreme Doubling. These potions are sold in the Auction House under 'Special'. These potions once bought are bounded and cannot be traded or given away. Now, this gets tricky, as you will want to determine how much extra attack/damage you will need to 1-hit.

Level 1-40 CA : 11 stam / round of attack (Level 40 = 10% extra attack/damage)
Level 41-80 CA : 12 stam / round of attack (Level 80 = 20% extra attack/damage)
Level 81-120 CA : 13 stam / round of attack (Level 120 = 30% extra attack/damage)
Level 121-160 CA : 14 stam / round of attack (Level 160 = 40% extra attack/damage)
Level 161-175 CA : 15 stam / round of attack (Level 175 = 43.75% extra attack/damage)

Compare ::
Level 165 Wither : 16.7 stam average / creature
Level 350 Wither : 13 stam average / creature

11x doubler
11x doubler comes from the Potion of Acceleration. This is a potion you can make from a recipe. Many level 400 players opt to get level 36 counter attack, making this setup probably the most useful, especially if that 9% extra attack/damage allow you to 1-hit. Also, note that since you will be 1-hitting, you can consider the suicidal setup and forgo defence and armor, but do remember, you can still miss (2%) or creature can dodge, and you can die. But the 1-hitting vs 2-hitting pay off is so great that usually it is worth the risk, especially if you can find some place to stash the gold as you hunt.

Level 1-36 CA : 12 stam / round of attack (Level 36 = 9% extra attack/damage)
Level 37-72 CA : 13 stam / round of attack (Level 72 = 18% extra attack/damage)
Level 73-109 CA : 14 stam / round of attack (Level 109 = 27.25% extra attack/damage)
Level 110-145 CA : 15 stam / round of attack (Level 145 = 36.25% extra attack/damage)
Level 146-175 CA : 16 stam / round of attack (Level 175 = 43.75% extra attack/damage)

Compare ::
Level 165 Wither : 18.37 stam average / creature
Level 350 Wither : 14.3 stam average / creature

16x doubler
16x doubler comes from the Potion of Greater Doubler. This is a potion comes from scavenging, and it used to be 11x. HCS made a recent change and it is now 16x.

Level 1-25 CA : 17 stam / round of attack (Level 25 = 6.25% extra attack/damage)
Level 26-50 CA : 18 stam / round of attack (Level 50 = 12.5% extra attack/damage)
Level 51-75 CA : 19 stam / round of attack (Level 75 = 18.75% extra attack/damage)
Level 76-100 CA : 20 stam / round of attack (Level 100 = 25% extra attack/damage)
Level 101-125 CA : 21 stam / round of attack (Level 125 = 31.25% extra attack/damage)
Level 126-150 CA : 22 stam / round of attack (Level 150 = 37.5% extra attack/damage)
Level 151-175 CA : 23 stam / round of attack (Level 175 = 43.75% extra attack/damage)

Compare ::
Level 165 Wither : 26.72 stam average / creature
Level 350 Wither : 20.8 stam average / creature

21x doubler
HCS gave a bounded item, Christmas Present 2008. It is a level 1000 doubler with 240 mins duration.

Level 1-19 CA : 22 stam / round of attack (Level 19 = 4.75% extra attack/damage)
Level 20-38 CA : 23 stam / round of attack (Level 38 = 9.50% extra attack/damage)
Level 39-57 CA : 24 stam / round of attack (Level 57 = 14.25% extra attack/damage)
Level 58-76 CA : 25 stam / round of attack (Level 76 = 19.00% extra attack/damage)
Level 77-95 CA : 26 stam / round of attack (Level 95 = 23.75% extra attack/damage)
Level 96-114 CA : 27 stam / round of attack (Level 114 = 28.50% extra attack/damage)
Level 115-133 CA : 28 stam / round of attack (Level 133 = 33.25% extra attack/damage)
Level 134-152 CA : 29 stam / round of attack (Level 152 = 38.00% extra attack/damage)
Level 153-171 CA : 30 stam / round of attack (Level 171 = 42.75% extra attack/damage)
Level 171-175 CA : 31 stam / round of attack (Level 175 = 43.75% extra attack/damage)

Compare ::
Level 165 Wither : 35.07 stam average / creature
Level 350 Wither : 27.3 stam average / creature

Now that we have so many different doubler level, I thought we should have a table that compares them. Comparing stam is unnatural, so we have to divide that by the doubler multiplier, since the xp given is multiplied by that. That'll give us the xp per stam compared to straight 1-hit. I also added comparisons for staying back one level, for those tough levels that may require high level of CA.

Buff \ Doubler	3x	4x	7x	10x	11x	16x	21x	Bonus
CA19							1.05	4.75%
CA25						1.06		6.25%
CA36					1.09			9.00%
CA38							1.10	9.50%
CA40				1.10				10.00%
CA50						1.13		12.50%
CA57			1.14				1.14	14.25%
CA72					1.18			18.00%
CA75						1.19		18.75%
CA76							1.19	19.00%
CA80				1.20				20.00%
CA95							1.24	23.75%
CA100		1.25				1.25		25.00%
Back level (1-hit)	1.25	1.25	1.25	1.25	1.25	1.25	1.25	NA
CA109					1.27			27.25%
CA114			1.29				1.29	28.50%
CA120				1.30				30.00%
Wither 350	1.30	1.30	1.30	1.30	1.30	1.30	1.30	NA
CA125						1.31		31.25%
Back level (CA25)						1.33		NA
CA133	1.33						1.33	33.25%
CA145					1.36			36.25%
Back level (CA36)					1.36			NA
CA150						1.38		37.50%
CA152							1.38	38.00%
CA160				1.40				40.00%
Back level (CA50)						1.41		NA
CA171			1.43				1.43	42.75%
CA175	1.67	1.50	1.57	1.50	1.45	1.44	1.48	43.75%
Back level (CA72)					1.48			NA
Back level (CA75)						1.49		NA
Back level (CA100)						1.57		NA
Back level (CA109)					1.59			NA
Back level (CA125)						1.64		NA
Wither 165	1.67	1.67	1.67	1.67	1.67	1.67	1.67	NA

So what's really better? Is it better to use 11x or 16x doubler? I've been asked this question quite a lot, so perhaps I should write it down here. My typical hunt usually use about 1800 stam in hitting, and 150 stam in walking while using 11x doubler. Yours may differ. If CA is not needed, then clearly, the 16x doubler is better. I would have used only 103 stam in walking, saving 47 stam. If CA is needed, and CA25 works, then I would have used CA36 and 1964 stam hitting, while CA 25 will use only 1913 stam, saving 49 stam (in addition to the 47 stam in walking). If CA 36 is not enough, and CA 50 works, then clearly 16x doubler is better again. Now, the tricky part comes when CA 25 is not enough, but CA 36 is enough. With 11x and CA36, I'll still be using 1964 stam in attack, but with 16x and CA50, I'll be using 2025 stam in attack. So I use 61 stam more with 16x, minus the 49 stam saved in walking, I still use 12 stam more. But wait, there's one more saving grace. I would have hit fewer times with 16x, so I will use less blocks on an imp. I'll say I use 1 block less, worth 8 stam, so I am down 4 stam. Unless all my levels are as such, that 4 stam probably don't justify the cost of changing doubler potion between levels. Another intangible on the 16x doubler is the time you need for your hunt is reduce by 30%. That could mean to some, no rebuff, or fewer rebuffs.