As Magicians we are likely to come under attack by lesser Spirits, Parasites or even other witches from time to time. This is why it’s important to have a Spirit as a protector to help you in this area especially as a beginner.
This Spirit can be an Archangel, Archdemon or even a Pagan Deity. Even a Thoughtform.
It seems very beneficial as they can negate the attack and then deal with the attacker as they wish.
agreed! That’s why it’s important to make lots of friends!
I always enjoy the terms you use! 
From my own findings servitors work best for protection, for me at least.
But couldn’t we also be attacked by higher spirits? For instance, someone summons one to attack us or we offend one in some capacity.
Then our deity will step in.
Generally offending one will not result in them attacking you, you may be reprimanded or they may not work with you ever (probably the worst punishment). I don’t think I’ve heard of anyone angering a Spirit to the point were they had to kill that person…
Also some Spirits attack you to test you or purify your energy.
Of course, because of survivorship bias. Anyone who was murdered by a Spirit, of course you wouldn’t hear about it, because they’re dead.
There are probably plenty of people who are murdered by spirits because of a serious offense.
Bog Veles and Bog Perun punish oathbreakers with death.
Slava Bogum
Everyone’s been murdered by spirits before! If you’re familiar with past life regression therapy, and are good enough at it to ritualize it, you can tune your look-backsies to the magical patterns and spirits at work on and under the surface, and sort by lifetimes of influence. Since everything has a form of a consciousness, you can pretty much always discover a Nemesis this way- a you-killer, that is! 
Could this explain why one would be somewhat afraid of working with a specific Spirit?
Yeah, OP! It also can form specific phobias and weird life reoccurrences, too. It’s all dependent on how your energy reacts to the death!
Fear is awareness, so yes. They’re subconsciously aware that that individual is a threat.
Michael is said to be the most powerful protecter in the universe. Is Lucifer is Michael’s equal when it comes to being the most powerful protecter?
In my opinion, Michael has a slight edge over Lucifer as far as protection goes, but Lucifer is a powerful and respected being, and there are few things willing to face off with him, so he is still an excellent protector.
In the context of magical attacks?
Yes, you can call upon Lucifer for any kind of protection. Like I said, he is powerful, and respected.
I have considered Lucifer a lot but it’s not good if there exists magic that can break his shield…