@Anonymous Hi,
The only thing I can think of is to run the flow each week on Sunday and then use a number of conditions to check if it is the last Sunday of that month. Then put any actions you wish to perfom under this condition.
For example you could do the following

Formula for 'Current Day of Month'
int(dayOfMonth(utcNow()))
Formula for ' Current Month'
int(substring(utcNow(),5,2))
Then use a condition to see if the current month has 30 days. If yes, then use a second condition to see if the current date is the 24th or later in the month (the last sunday of a month with 30 days must be on the 24th or later)

**Correction** the condition 'not equals to: 02' should be 'not equals to: 2' as the value is converted to int there is no leading 0
You can then repeat this condition for months that have 31 days and for feburary for 28 days. I recommend putting each of these 3 master conditions in a linear fashion rather than inside each other to make for a cleaner flow.
For months with 31 days, the last sunday should be at least the 25th of the month and for feburary it should be at least the 22nd of the month.
Hope this helps,
Sam