Here is a very simple chart displaying the type match-ups of the Light type.

There is reasoning behind it, as to why I have chosen it to be as it is.
Firstly, it is a balanced type: 8 types in the positive aspects (strong vs + resistances + immuities) and 8 negative ones (resisted by + weaknesses).
Secondly, there is some logical sense to it.

In the "strong vs" category:

• Light dispels dark.
•  UV light causes objects to lose charge.
• Most bug seek the darkness, and the whole burning ants with a magnifying glass thing. 

In the "resisted by" category:

• Grass and Ghost (as something that creates shadows) absorb light.
• Light and Psychic are themselves or use the electromagnteic spectrum, so are 'used to it', in the same way fire resists fire, or water resists water.
• Steel and Ice both reflect light.

In the "weak to" category - as above, Grass and Ghost absorb Light, so would drain Light pokemon's energy.

In the "resists" category - Electric, Fire, Light all produce Light, and again the EM spectrum argument for Psychic attacks. The immunity to Dark should be self-explanatory.

I understand other people will have different interpretations, I have found other people make Light fakemon, with sometimes opposite match-ups to mine. However, I like mine, as It pulls back the some-what overpowered Dark type, and the only one weakness Electric type, and pumps up the Grass type (statistically the worst in type match-ups) and the Ghost type, which I felt got a bit trumped by the Dark type as Dark is super effective against the same two types, but also resits/ is immune to them; whereas ghost isn't. Moreover, Psychic and Ghost pokemon often have lower defence than special defence, so the abundant physical Dark type attacks with pokemon with high attack stats would be better again, than the standard Shadow Ball.

But anyway, for my Fakemon, the above chart is the law for type Match-ups. Meaning the Light type comes across as a more defensive type than offensive, which seems to suit.