The best Magcro rotations are very dynamic and difficult to master. I’m not a great Magcro so I just do a simple one.

Front Bar: Skull, Blastbones, Arcanist, Siphon, Inner Light, Shooting Star

Back Bar: Unstable Wall, Orb, Boneyard, Channeled Accel, Siphon/Flex Spot, Colossus

Rotation is to start with: Channeled Accel, swap, Blastbones, Arcanist, swap

Then repeat:
1. Boneyard
2. Orb
3. Wall
swap
4. Blastbones + Siphon (simultaneously)
5. Skull
6. Skull
7. Blastbones + Siphon (simultaneously)
8. Skull
9. Skull
10. Blastbones (no Siphon here, save the corpse for Boneyard)
swap

Recast Arcanist instead of a Skull whenever it runs out, and use Colossus instead of Skull as needed. If the fight is long enough to recast Accel it just replaces 2 Skulls in a row, or you can use Barbed Trap dynamically replacing one Skull cast. You have to be pretty close to the enemy for the Blastbones to work with only 2 skills in between, but this is important for damage and corpse generation.

That setup has 4 Gravelord abilities front bar so it works best with 60% Spell Crit. I like using Spell Strat, False Gods Devotion, and Ilambris. There are many other sets that give similar results: Sorrow, BSW, Siroria, NMA, etc. If you have closer to 50% Spell Crit then replace Shooting Star (only there for the Mages Guild passives) with Colossus on both bars for more crit in execute.

For my Magcro I really like using Draugrkin + Mother Sorrow with maelstrom staff back bar (double lightning destro) , but since you are new and might not have access to these sets using sorrow and julianos is a great mix!
For my skills I use Ricochet Skull, Blast Bones, Arcanist, Mystic Siphon, Degeneration, and Colossus on my front bar, and then Unstable Wall of Elements, Boneyard, Summoner's Armor, Mystic Orb, and Channeled Acceleration on the back bar (Channeled Acceleration is a Psijic Order skill gained through the Summerset expansion, you can replace this with trap from the fighters guild for minor force if you dont own Summerset!) and my rotation usually goes like this:

Prebuff with Summoner Armor, Arcanist and channeled accel, cast blast bones as you lead into the boss fight then start with the back bar doing LA > Wall > LA > Boneyard > LA > Orb > Swap > LA > Blast Bones > LA > Skull > LA > Degeneration > LA Arcanist then immediately mystic siphon (siphon is off the gcd so you can do this with ease) > LA > Blast Bones > Swap to back bar and repeat
LA = Light Attack
Use Ult as soon as you can, I reccomend using witchmother brew for extra recovery if needed
The sets mentioned in other posts are ones id reccomend as well, with the addition of Draugrkin (in the Harrow Storm DLC Unhallowed Grave Dungeon) to really boost your damage over times abilities, specifically mystic siphon, which procs multiple times a second!

Dynamic rotation is the key. Your priority is Blastbones.
Before engaging the fight don't forget to pre-cast Skeletal Arcanist, Channeled Acceleration and pre-summon blastbones. It should hit the target the moment tank pulls the boss and then you drop the graveyard on top of its corpse to get that 20% bonus damage. You know the rest.