Guys beating tank 1 on 1 is simple with enough strategy and practice with it.
I went up against a tank one on one and he missed at least 6 or so punches (during our UC and friend no pubber game) (In the end a smoker or jockey got me, I can't remember, I got clipped onto a wall and couldn't dodge that tank punch)
To do this do the following.
1)Spamm auto shotty till bullets are out.
2)Pull out melee.
3)When tank punches jump to the left or right.
4)The timing is a bit tricky, it has to be a tiny delay after the other player presses m1.
5)Also, try to jump towards what arm is punching as if you do so during when the tank raises the arm but didn;t slam it down/hit you yet, you can clip through or atleast it won't register damage.
6) I'll try to find a video of this:
http://www.youtube.com/watch?v=CesrRAVb9jIOk this isn't exactly it but it's kinda close, You can dodge all the swings, by jumping to the side.
Things to watch out for:
DO NOT JUMP BACK; Jumping side+back more often then not won't work, because you either underestimate the length of the arm or he'll catch you with the second punch, or you'll soon hit a wall behind you.
Try not to rely on this heavily until you have it perfected, instead use this to quickly escape a tank and run back from it.
Watch the fuck out for smokers and jokeys

Example:
--o-- = Tank
x = you
Fig 1 :
I
\o--
X
As you can see the tank has raised his arm. This occurs right after the tank presses m1.
Jump to these locations quickly.
I
\o--
X
X