It seems English A level may prove to be pretty useful for this... Descartes' Ontological Argument.

Firstly, something that is worth noting about Descartes, is that as a sceptic, he approached his arguments from the point of view that he is proving that God cannot be doubted. This was because he always worked towards an end goal, by starting off by doubting everything at first. Which is where his famous quote "cogito ergo sum" or "I think therefore I am" came from, as in this case he used scepticism to make a point about whether he even existed himself.
  

So what did the dubbed 'father of modern philosophy' have to say about God? Well his argument is as follows...  Firstly alternatively to Anselm's description of God as that than which nothing greater can be conceived, he says God is the supremely perfect being. So by stating this at the beginning, it already shows God to be a being, and therefore negating the need to change him from a concept or an idea into a being at a later stage in the argument. His second point is that a supremely perfect being has all the perfections that there can be. Indeed by giving God attributes, it is therefore enforcing this idea of God as a being, rather than a whishy washy concept or idea. Thirdly, Descartes says that existence is a perfection, because similarly to Anselm's argument, being in existence is better than not, therefore making it a perfection. And lastly to end his argument he says therefore God exists necessarily. Now an alternative way that he could have ended his argument which would have saved it from contradicting itself, would have been to say therefore he possesses the quality of existence. This would have ensured that he did not confuse God to exist (verb), and God to have existence, (abstract noun), and therefore not have stated him to have two differing attributes. The problem within these arguments, is that when people such as Descartes use both the verb and the abstract noun to describe God, it is just completely incoherent. For, the definition a verb is a doing word, so it would not make sense to say that God has 'exist'. Which is what he starts the argument off by saying, but by replacing the word exist with existence. So again, if he had altered the last point of his argument it would be coherent, as it would just show God to possess existence.

Despite this, another part of his argument that is clear to us, is that he sees existence as something that God has, which makes existence a predicate to God. This means that it is a quality that God possesses and affirms it as an attribute of him. He gives the example of triangles, and mountains. In these examples he says that you can think of a triangle only as a shape that has 3 interior angles that add up to 180 degrees. And a mountain can only exist if there is a valley along side it as well. However, although it is true that you can imagine these two things not actually existing within reality, it is impossible to think of God as not existing, as he is entwined with existence like a triangle is with having three angles.

I thought I was on to a winner with this argument. Sadly not so! Mulla Sadra's Ontological Argument

Whilst researching Descartes' Ontological Argument, I stumbled across an Iranian Islamic philosopher who apparently led the Iranian cultural renaissance of the 17th century. And that's not all, he has also been described as arguably the single most important and influential philosopher in the Muslim world in the last four hundred years. Impressive stuff! So after reading this and glancing over his argument, I thought that he may have dodged the grammar bullet that seems to be taking out a lot of these Ontological arguments. However, that is sadly not the case, and the words 'exist' and 'existence' still appeared within the same argument. Therefore still posing the same problem to the argument as that of Descartes'. 

On the contrary, something I found particularly interesting about Sadra is that unlike Descartes and many other philosophers, his argument comes from the stand point that he is not working a priori and in fact rejects these kinds of arguments on the basis that existence precedes essence. And also that the existence of human beings is more fundamental than their essence. So, to challenge these ideas Sadra put forward an argument called the Argument of the Righteous. The argument attempts to prove the existence of God through the reality of existence. In addition to this the argument proves that a thing is demonstrated through itself, which perhaps reinforces the idea of a necessary God. 

Now, to the argument itself... He starts off by stating that there is a being. Then follows on by saying that this being is a perfection beyond all perfection, and that God is perfect and perfection in existence. After this is actually what caught my eye in the first place with his argument as he goes on to say existence is a singular and simple reality. I found this to be a very unusual way of describing it, as I have not yet come across anyone else who describes it as such. In addition to this he continues to state that singular reality is graded in intensity in a scale of perfection. And finally, that scale must have a limit point, a point of greatest intensity and of greatest existence... And that is when I saw it. The issue with this argument, just like many others. Because unfortunately that was not the final point, and as you guessed, he finished by saying "Therefore God exists".

In doing this, he like many others, jumps from God having existence, to being a God that exists. So by having an unfortunate important incoherence it does open up this argument for criticism. But despite this I found his argument to be an interesting alternative to Descartes' as I like the idea of a scale of perfection, and the way in which everything is presented as singular and together in one reality. Furthermore it also reminded me of Irenaeus' point about our potential to reach perfection throughout our lives, and it gave me an idea that perhaps we slide along Sadra's scale until we reach our own intended desired perfection.

It's Anselm not Anslem, and definitely not Aslan.... Anselm's Ontological Argument version 1 + 2

Fides Quarens Intellectum. Anselm was a man of faith, and wrote from a position of faith. Through this Faith Seeking Understanding he would have been writing to put across his Ontological Argument to other people of faith. And this possibly is the most important factor to remember about the argument, as he was starting from the Christian understanding of what the definition of God is. Something that Anselm sought to do with his argument was challenge the fool, who is written about in Psalms 14:1 and 53:1. In both instances it says "The fool says to himself" "there is no God".

Now the first thing to understand is the definition of God that Anselm uses for this ontological argument. For him, God is that than which nothing greater can be conceived (TTWNGCBC). So seeing as this was what Anselm's idea of God was, this also applied to what the Fools idea of God was. So in saying "there is no God" it would seem that the Fool has somewhat contradicted himself in the eyes of Anselm. Indeed as Anselm continues to explain, both existing in reality and in concept is far greater than just existing in concept alone. Which therefore follows that existence of any kind is a great attribute. And as God is that than which nothing greater can be conceived he must therefore fulfil his own definition of being the greatest by existing in both reality and in concept. So, going back to the Fool, it would seem that because he has an idea of God, he is presumably in understanding of the definition of God along with his idea. This is due to the fact that he cannot imagine something and then contest the idea without sharing the same definitions. So because he has an idea of God, by definition this God must exist, for as Anselm explains, God is that than which nothing greater can be conceived.                                                                Which means God exists!

However... Gaunilo had a different idea... For he gave an argument against Anselm's by giving the idea of replacing God with the an island of your dreams, or if you like, that than which no greater island can be conceived. And he seemed to prove Anselm's argument wrong for a while as he correctly said that just because he can imagine a perfect island does not mean that it will exist in reality. As well as this, Gaunilo also stated that "An object can hardly or never be conceived according to the word alone" as the word corresponds to an idea, and while the word may stay the same the idea behind it can be changed endlessly.

BUT! Anselm wasn't so easily proved wrong and came back with a more refined argument. In addition to that, he also reminded Gaunilo that anything that you apply to the island is going to be contingent, as they are all Earthly features, as opposed to attributes of God which are necessary, and cannot ever be understood fully by people. So in order for Anselm to refine it, he asked the question, which is greater; A God who can be though of as not existing? Or, A God who cannot be thought of as not existing? His answer was the second question, because it presents an idea of a God where it is impossible for God not to exist. Furthermore, not only is the second question a description of a greater God, but again a description of a necessary God, therefore proving Gods existence as necessary.