This is a easy guide on HOW TO FARM WITH THE PLAGUE RAT
i am main twitch played it from level 1 to 30
i also used to jungle with him BEFORE kindred and actually got a pretty decent plays with him has jg
first penta at level 5 with twitch

you cant actually be...
1 agressive (dont recommend unless you and your support have a call going)
2 play passive (but then you can be poked and zoned out of the lane or worst getting kill

TELL YOUR SUPPORT THAT STAYS NEAR YOU OR FRONT OF YOU (well if its a tanky supp or agressive)
how to avoid ganks
use your Q and run.......
use your Q and go to a bush and BEFORE you turn invisble push B to start a teleport but you will still be invisible!

For ganking you need to have Blade of the ruined king,dancer and infinity.
your standar will be Q a very good distance from the lane so the common ward wont spot you,
then stand behind the enemy or behind (if he already engaged)your ally your standar combo will be blade of the ruinend king then W then auto attacks till he tries to run away or he has less than 1/4 of his health bar then use E or you could use it when they reach the 6 stacks!
or the another combo
Q-W-R- auto attacks till he is dead or he tries to get away if he got away and he is on turret use flash to get in and use E to kill him (if he has low health)

you are an assasin so your targets are: ADC, APC (mid) or support if they are like soraka or janna.
the same tactic has before use Q go behind the enemy team were they are and use R and start shooting at them!
you can kill 2 of them if they are slow to response or you cant make them recall breaking the TF or even better damage all of them so you team can get them all!.
last tip runaans bow + ulti means if you hit a target that is behind your main target and gets hits by the runaan bolt means that you will damage them for 25% +80% of your attack (or 160% +25% if crit) this will make an INSANE AMOUNT OF DAMAGE TO THEM.

PROS
you can get hits of 1000 of damage
you can get their adc mid or support very easy
you get in 3 or 4 AA = dead

Cons
Low health
Low