Twitch is highly damaging champ. It can provide long lasting damage which can help you secure kills if you died. Because its venom its damage get longer depending how many stacks is on enemy champ.

For I suggests that focusing on damage.
Greater glyph of scaling magic resistance help you get save from magic damage initial to final levels.
Greater Seal of Armor gave you high armor so you can able to absorb more damage.
Greater Mark of Attack Damage gave you desired initial damage which Twitch needs.
Greater Quintessence of Attack Damage help you more in producing damage.