If I have 5 creatures and you have 2, I will always be the last to activate a creature unless you kill my last creature or otherwise prevent it from acting (Banish comes to mind).
5 2
you pass, I go
4 2
you pass, I go
3 2
you pass, I go
2 2
you go
2 1
I go
1 1
you go
1 0
I go
Whether you have initiative or not, with with more creatures you will always have the final activation.
As for attack spells, many attack spells will be expected to kill the Invisible Stalker for a similar amount of mana to the original investment (2 Lightning Bolts, 3 Arc Lightnings, 3 Jet Streams...), but the thing to keep in mind is that your opponent can improve those attacks. With Hawkeye, Lightning Ring, and Akiro's Favor, you shouldn't need more than 2 Arc Lightnings. Pillar of Light and Invisible Fist are also pretty efficient against Invisible Stalker even without support. Sure, improving ranged attacks require some setup, but that gives long term benefits that are often worthwhile anyway for the strategies that run them. In addition, stunning the Invisible Stalker is particularly efficient due to its upkeep.
Anyone running a couple of Pillar of Lights or Invisible Fists is in a good position to counter the Invisible Stalker efficiently, but Wizards counter Invisible Stalker much more efficiently. It's not that Wizard's are OP in general, it's that Arcane Zap hard counters Invisible Stalker.