Need Help with Dynamic SplitColumn

Assuming there is just a single column that contains a colon, you merely need to find that column:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("JczLDQQxCAPQXpDmxiF8w9BKlP7bmMV7MbL1xDkkxCQTquv5HfMO1OKky4d0wIQagFdvm+1lB0CZUAfIalnzIhcHhI+Y0IAQ2S07sTobTEyb0PybstaNtYKF7v0A", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Level = _t, ManaCost = _t, Damage = _t, ColdDamage = _t, ColdDPS = _t]),
    
    #"Split Column" = [a=Record.FieldValues(Source{0}),
                       b=List.Transform(a,each Text.Contains(_,":")),
                       c=List.PositionOf(b,true,Occurrence.First),
                       d=Table.ColumnNames(Source){c},
                       e=Table.SplitColumn(Source,d,Splitter.SplitTextByDelimiter(":"),{d & "_Low", d & "_High"})][e]
in
    #"Split Column"

Sample Data:

Results: