People tend to think that Singed is a bad champion to use but that is not true.This guide is mainly for 3v3 I wouldnt recommend it for 5v5.With this guide you can do 200+ with poison and 800+ with fling, you can easily take down a squishy within 3 hit in late game.

Singed descended from a long line of Zaun's revered chemists. Even in his youth, his talent for concocting potions far outstripped that of his peers, and he quickly distinguished himself from his less extraordinary chemist compatriots. It came as no surprise to anyone when he was selected for apprenticeship by the infamous Warwick, master apothecary on a lucrative retainer with the Noxian military during their campaign against Ionia. Within Warwick's laboratories, Singed toiled without end, rapidly absorbing every detail of his predecessor's deadly craft. Singed had little concern for the death and destruction that was the fruit of his labors. By the time the curse of lycanthropy descended to claim his master, Singed was poised and eager to make the transition from workhorse to innovator; he was ready to share his genius by bringing a new brand of suffering to the Ionian front. His zeal for progress was unquenchable, and when suitable test subjects proved to be in short supply, the eager chemist was often thought to turn his volatile mixtures on his own flesh.
When the uneasy peace created by the League of Legends settled on the world, Singed journeyed to the one place where he was still able to showcase his beloved craft: the Institute of War. By this time he was barely even a man, his body both ruined and sustained by his ingenious craft. A thousand burns - accidents of shadow and flame - mar his ravaged form, and exposure to such harsh conditions has deadened his nerves, hardened his body, and strengthened his physique, transforming him into a veritable juggernaut. This, combined with a formidable arsenal of deadly concoctions, make Singed a force to be reckoned with on the Fields of Justice.
"My deadliest dose shall bear my patron's name!" –- Singed, having just christened the Insanity Potion

Pros

-Good Speed
-Tanky
-Powerful Slow
-Nuke
-Can take two people on at once easily
-Powerful Poison
-Can chase

Cons

-Very mana dependent in early game
-Can't jungle
-Slow attack speed
-Slow at destroying turrents

Ghost
I think Ghost is a must have when using Singed.You can use it to run away,or chase.When using using Ghost and Insanity Potion together if gives you incredible speed to run away from your opponent or when chasing them.

Flash
Flash comes in handy in many different situations.You can Flash right out of other champion's reach when you have low life or when being pursued.What you can also do with Flash through walls.

Exhaust
Exhaust is another good summoner spell to use. You can Exhaust to catch to your opponent to fling them back into your Poison Trail.

Ignite
I get ignite when my team doesn't have an ignite ignite is really good on champs that use heal or champs like Tryndamere and Swain.

Poison Trail - Leaves a trail of poison behind Singed, dealing damage to enemies caught in the path.
Toggle: Singed lays a poisonous trail that deals 22/34/46/58/70 (+0.3) magic damage per second.

Mega Adhesive - Throws a vial of mega adhesive on the ground, slowing enemies who walk on it.
Leaves a sticky area on the ground for 5 seconds, slowing enemies in the area by 35/45/55/65/75% (lasts 1 second after leaving).

Fling - Damages target enemy unit and flings them into the air behind Singed.
Singed flings an enemy over his shoulder, dealing 100/150/200/250/300 (+1) magic damage.

Insanity Potion - Singed drinks a potent brew of chemicals, granting him increased combat stats.
Singed drinks a potent brew of chemicals, granting him 35/50/65 Ability Power, Armor, Magic Resist, Movement Speed, Health Regeneration, and Mana Regeneration, as well as reducing the duration of disables by 10/15/20% for 25 seconds.

Empowered Bulwark - Increases Singed's Health by 25 for every 100 Mana he has.

This is your ultimate it give you CC reduction, magic resist, armor, ability power, and movement speed.Activate your ult whenever your going into a fight or when running. I wouldn't recommend you use Insanity Potion when chasing UNLESS your sure you can take the champion down.

This will be your main farming move, and it will be your secondary killing move.In mid game if you want to push you can kill minions in a few seconds with poison. If you get Rylai's Crystal Scepter you will also slow with your poison.

In mid/late game this will be you main killing move/move to initiate with , you can combo this with Mega Adhesive by using mega adhesive the flinging them back into your Poison Trail. This Skill will be really powerful after you get your Lich Bane (remember to auto att once after fling!)

You can put this infront of your oppents to slow them, then you can follow up with your fling. If you ever need to escape you can also activate it right infront of you.

Im am not quite done this guide, though im just showing it to everyone for some feedback. Thanks All!