Let the bodies hit the floor

Gem of iso is usually core on him.

The leveling order described in this guide is a bit screwy, and not accurate as of 5.13.

Now that Undead Surge provides 2 corpses even at level 1 (instead of level 3 like it used to), it's now likely better to concentrate on your Corpse Explosion and Fleeting Breath first, due to their higher damage potential. You'll start with Undead Surge, but probably won't put any more points into it until later.

You'll probably also consider delaying further levels in Empty the Crypts (except 1 point at level 5) until you've gotten your other abilities up.

15 pen is good, but you may not understand the relationship between damage and amount of protections. As protections get closer to 0, your damage increases in a curve, not linearly. You want more than 15 pen in a complete build as a damage god, because enemies, at level 20, will have a minimum of around 48 protections even if they build no protection items.

I can see how you don't find Spear of the Magus to be helpful. The way you've described your gameplay style is a simple one, using the basic 1-2 combo (and yes, doing one right after the other is actually extremely common...you always try to burst right after the corpses hit max range or the enemy). This doesn't get much out of Magus' stacking protection reductions.

To get the most out of Magus, when you recognize a situation where you can get a kill, you actually lead with Fleeting Breath. If you also get Gem of Isolation, you can stay close to them even if they're retreating due to the slow...that slow, with the ticking damage, will last 6 seconds, which is extremely long.

Once you've gotten multiple ticks, THEN you unleash your 1-2 combo. Against squishy gods that built 0 protections, even at level 20, you'll deal true damage (no mitigations), and that damage is going to be significantly higher. Thing is, you have to know how to use Spear of the Magus if you want to get the most out of it.

It also works well with your ult. Ah Puch can be a juicy target, due to his lack of escape. Save your ult for when people try to gank you. Immediately pop it...with Gem's slow, you should be able to create some distance, and all the while, Magus will be stacking on that enemy. Turn and blast with the 1-2 for high damage.

Also, I personally find Chronos' Pendant to be extremely helpful. Sure, his CD's are fairly low, BUT...if you're spamming your abilities, the passive extra 1 second CD every 10 seconds is always going to help. And you'll have your ult up significantly sooner than if you didn't have it. It's not necessarily core, but it's very, very nice to have.

(FYI, 10% extra damage from Shaman's Ring isn't all that much. If you deal 500 damage, that's only an extra 50. If you were to get Chronos instead, I'm pretty sure you'll be able to deal more damage overall, if you're consistently connecting with your abilities)

If you're interested in learning more about penetration and protection reduction, check out this chapter of my Items guide.

"spear of magus and found it to not be as useful as the other items" - get Obsidian Shard then. You need some flat pen in your build and some percentage penetration to tackle tanks (guardians, warriors etc...)

And if you are using him correctly then Spear of the Magus is good. Thow your 1,3 and then your 2. Remember your 3 and ult are damage over time abilties.