As I see it now, plot is written first, and logical explanations are added later. So, charachters behave very suboptimally.

What would you do as a Bolas ? Elesh ? Jin-Gitaxias ? A random competent planeswalker willing to conquer the Multiverse ?
What would you do as a Jace ? Sorin ? Liliana ? Tamiyo ? A random competent planeswalker willing to help the Multiverse ?

Plant spy networks ? Deal with authorities ? Some mind control spells here and there ? Artificially create, torture, and destroy souls en masse until you get a million sparks ? Bond with lands until you get enough mana to do anything ? Transform yourself into a shapeshifter, and steal a form of Bolas and be an immortal mind-shattering superdragon ?

Research spells and teach people to use them ? Spread artifact blueprints ? Make a nice, family-friendly version of Oil ? Establish mass testing of people on sparks, and create planeswalker-training camps ?

I don't see the problem with this, specifically the idea that characters behave suboptimally.

I could be an Elesh Norn and create a peaceful, soulless multiverse

Right now? I've seen a lot of plots, and of course which one works depends on the assets and skills of the characters involved. Nicol Bolas can be very straightforward, because part of his thing is being the top dog of the Multiverse. Vraska needs to be a little more devious. OK, a lot more devious, that plays to her strengths. Are they going against the Gatewatch? Or Urza? That takes a different villain. Similar, heroic strategies are very different for fighting Bolas and Fighting Phyrexia.

That said, here are two speculatives.

How would I conquer the multiverse?

Subversion. Would-be tyrants lose because there exist those with the will to resist them. Misinformation, Propaganda, False orders... these are the tools that spread Disharmony and cause my foes to Turn Against each other. Would-be heroes like the Gatewatch wouldn't even know my name -- not as a foe, not until it's too late and they face Insurrection while all the while dancing on my Puppet Strings. After all, it's easier to Divert such a force than it is to stop it: keep their suspicions directed elsewhere, seed betrayal, foster Hatred, hide behind the Shadow of Doubt, and their Tragic Arrogance will cause their Worst fears to become reality.

In short, make my enemy's strength into my strength.


How would I save the multiverse?

Knowledge and Unity are key. You can't fight a threat you can't understand, and you don't want to do it alone. Solidarity makes the Multiverse stronger than any one foe; this has been proved time and time again from the Victory of the Coalition to the success of the Gatewatch on Zendikar. If we're going to save the multiverse, we've got to make it Better for everyone. This is a Collective Effort and we need to Join Forces. As a Planeswalker this is going to be particularly hard: some situations won't want to be saved, and you can't drag them to salvation kicking and screaming, you have to win hearts and minds. Directly overpowering the opposition isn't likely to win and if it does, that kind of victory won't last, nor will it make you any better than the likes of Bolas.

In short, identify the problem and enlighten and organize those affected.

Something I learned from competitive games is that, if you know that an option is available, the opponent can plan for that option and work around it.

Therefore, I would use whatever tools and tactics that I know wouldn't fail. If I see a particularly weak target I need dead, that doesn't have anyone that would know any better, I would engage in a Massive Raid using whatever is the strongest resources I have on hand to get rid of him.

Murder
Destroy the Evidence

Being able to refuel with more renewable resources is critical to my strategy. If I want immortality, then I also don't want my immortality machine to run out of gas. And if I'm saving or conquering the world, I know that running out of gas and relying on critical reserves can mean the end of my cause.

Many belligerents in the Multiverse tend to abuse powerful and dangerous weaponry, especially the incompetent idiots vouching for peace, too greedy to care, or even "freedom fighters". But for some reason we still love them, and because of that I would similarly use such force when necessary.

And also, find a nice copy of Ancestral Recall.