When playing Varus there is 1 and only 1 trick - stay at a safe distance for most of the time and harass the enemy. Always try to trigger the blight stacks, and never be scared to use your ult, even if it is not in team fight. Do NOT save your ultimate for team fights only that is one of the worst mistakes people make, which leads to performance issues. Varus is one of the carries who, despite lack of escape abilities has great CC, so use that to the max. In my opinion Varus has no actual counter, despite popular claims that mobile champions like Ezreal and Sivir could be his counter, he like Caitlyn is OP and has been for quite a while, only if played correctly though. And most important - get used to how much damage you can dish out - that comes only with practice so go try this out and see for yourself, it will be a bit harder at first, hell i was terrible when i first started playing him, but with some practice, now i main Varus. 'Nuff said - where there is will there is a way - train hard and snipe like a boss :D

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Pros:

Has EXTREME RANGE from Lvl 1
Is great DD
Melts Tanks
Scales on AD and AP, yet AD is absolutely enough
Steroid Passive
Crazy CC
Healing reduction

Cons:

Has no escape ability, but is not much different than many champs
Is slow early game
Slows even more down while charging his Q.

Concentrate on focusing all the Squishy enemies, one after the other, and if anyone tries to flee - quick Q and end his misery.

The best supports for Varus are:

Leona
Blitzcrank
Thresh
Alistar

Honorary mentions:

Lulu
Nami
Kayle
Taric
Janna
Morgana
Karma
Soraka

If you feel funky:

Fiddlesticks
Annie
Sion
Kennen
Cho'Gath

Most viable options are Flash and Barrier, and if you are Very confident in your skills you can go with Ignite as well. Flash and Ghost will give you bigger mobility and escape mechanisms, which make them also a very good option.