This guide is heavily geared towards making Willow the nuker she was made to be, and/or a reasonably powerful rapidfire auto-attacker.

Willow makes for a great burst mage assassin however you need to remember that you are a support/semi-carry, not a true core, not a braindead easy right-clicker with huge nukes, not a spam-happy nuker that can carelessly wander where she pleases; she's a glass cannon that more often than not relies heavily on her spells and item actives to be available - burning everything at once and exhausting your mana can and will leave you defenceless, and poor plays/positioning can and will get you killed in 2-3 hits, as without big health/survivability items you have little more effective HP than a creep.

Willow requires a good/careful approach to initiate and secure kills, good/lucky plays to make pickoffs, and often times the luck of the devil to get away clean.
Even if you try to build tanky, you're not goung to fare well in a face-to-face auto-attack fight with heroes that are more suited to that, so don't try it unless you're absolutely sure it's the right play - her auto-attacks are meant for farming creeps and stunned/low health heroes, not for face to face fights (unless you have some kind of damage immunity, i.e. OmniKnight).