Two rune spawns has made the game harder for Invoker because he needs levels to be effective. It is now more difficult to contest runes. Even with forge spirits, Invoker cannot guarantee a rune deny for bottle holders.

Lane Phase
Alacrity and Sun Strike will be your primary spells. Make sure you get tangos/clarities as needed. Always carry TP scroll after boots! Because Invoker has slow starting MS, try to secure top Rune, but don't force it. If mid opponent is gone, you can also Meteor creep wave and take tower with alacrity!

Early Game
Contest runes as you see fit. Avoid initiation at this point. Cold Snap, Meteor, & Sun Strike are your primary spells. Tornado is great if you need it to disable buffs (Ursa, Ember, Drums, etc). Ice Wall is your secondary spell. TP secretly as needed.

Mid Game:
Your moment to shine, but this build's primary goal is not to initiate a team battle. You are great at picking off heroes solo or assisting team with Sun Strikes. Nyx, Pudge, NS, etc are great allies for you (Use Sun Strike)!

Combos at this stage:
Urn + Snap then Meteor and Blast or SS
Eul + SS, Meteor, blast

Late Game:
Don't be afraid to use Alacrity on spirits before battle. Harass tower/racks with your summons. Your main job is crowd control, so don't stand in the front line unless you are initiating. Always have spirits before battle and pay attention to their cool down. Tornado/Meteor/Blast is a great combo, always consider it.

Very Late Game:
You desperately need to end unless you have a good hard carry. Invoker is a great late gamer, but he cannot compete with hard right clickers like Void, Medusa, Spectre, etc. Magic Immunity is also a big pain, but at least you have spirits, and alacrity. You even have tornado, blast, and ice wall to disable the ones without magic immunity.

For more details on spells, strategy, and important tips/tricks, see my comprehensive guide after 6000+ games at:
http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=185528211#

Demonstration: Match ID: 947536504
Votes/Comments are much appreciated.