I just did some analysis of this combo and unless I calculated something wrong then here are my findings:
Looks like a 9 mana combo minimum since you need to spend 1 mana to Vivid a 3rd Radiant. I can't see how it would be 8 mana (the reddit thread had people claiming it was 8mana).
There is actually LOTS of room to make the boar greater than 32/1 and you can also Vivid the Boar at the end to have 2 (or more) of them. You have 4 radiants (free mass dispell), 1 test subject, 1 boar... you can kill the last Test subject, gives you 5 minions on board (1 being boar), you can cast 2x Vivid to have 3 total boars after you already buff the 1 to 32 (or more) attack (use Vivid after the last Topsy).
You also have plenty of hand space to do the rotation a few more times, easily creating some monster damage
This is my theoretical board/hand state after the 4th rotation:
Mass Dispel has been cast during 4th rotation (no taunts in way).
Board: Radiant, Radiant, Radiant, Radiant, Test Subject (1x DS/Vivid buffs), Boar (1/4 (hit with DS 2x already

Hand: Divine Spirit, Divine Spirit, Vivid Nightmare, Vivid Nightmare, Topsy Turvy
Test Subject returns 2x DS/Vivids everytime you kill him as long as you DS/Vivid before topsy everytime. So you continually recieve 5 cards back.