Dimensional Tiering tends to be a bit wonky sometimes, but the image doesn't get that high into the power chart with what it has, to be honest. Derp is actually correct but he didn't get into much detail, specially given his choice in Tier 2, so let me explain.
Transcending "all dimensions" is such an incredibly vague statement, since dimensions are much more commonly used to refer to alternate universes in fiction and not the mathematical / geometrical ones that we use in VS Debating, so it can be interpreted as anything, just the word "dimensions" is not enough to place one as a higher-dimensional being, plus it says it can traverse through their space and time, giving more weight to this interpretation, rather than the higher-dimensional one.
As Derp said earlier, existence is just the total sum of parts that resides within a verse, since the verse is unknown, we have to stablish the size of existence based only on what we are told in here so we can give him a Tier corresponding to being "above existence" AKA being it's sovereign.
Now let's see... taking what we have:
* Traverse through space and time of every dimension = Hints at the dimensions being alternate universes that one can traverse through, likely hints at the ability to traverse them being important, if not, such emphasis would not be given in the first place, thus, having a separate space-time for every dimension and not all being together in one single space-time continuum.
* Transcended all dimensions = Hints at his superiority over the alternate universes, likely hints at him being on a state of being greater than the dimensions themselves, thus being superior to a space-time continuum.
* Above all of existence = Hints at him being the sovereign of the verse and above anything else within it, that includes every dimension AKA alternate universe, but since there is emphasis on the dimensional aspect and not the space-time aspect, this has him residing in a state of being above the space-time continuum within each dimension.
Conclusion: Big Chungus is bigger than a space-time continuum.
Proposed Tier: 2-C