Just pick Alistar like nothing special is going to happen, no one will expect you to deal tons of damage, they expect you to be a tanky dude but in reallity you are a bursty assassin.
-- THE SECRET ASSASSIN --

Make sure you know how to land your combo, its very crusial for the assassination, you also need to know when to NOT use the combo.

Don't forget your playing as the "Support role" soo make sure you don't kill your AD Carry while doing your assassination.

Even as an assassin, peel your AD Carry from bad situation, like when a Leona E's no him, just push her away like nothing happened.

Once you get LVL 6 you are allowed to do very risky plays, as your ulti almost gives you 7 seconds of "invicibility".

Try to roam, you have strong CC and strong DMG, you can do a very easy gank.

Inniciate TeamFights, by using ur W+Q combo on the enemy team and using your R to make you invicible.

Thanks for seeing my guide, let me know how it went out for you in the comments and ask me any question you like and let me know how can i improve this Guide!

---Alistar, The Secret Assassin ---