Mecha Phantom Beast decks are great at summoning multiple weak tokens you can use as Synchro material. These tokens they summon are level 3 making therefor you can use 2 Tokens along with a level 3 Tuner to Synchro summon Giganticastle.
You must have missed out something. Like "no special summon" challenge or something.
I'm using him with Destiny Heroes, usually works great when I normal summon a tuner while Anki or Vision Hero Witch Raider are already on the field. But twice now, when I had my tuner set on the field last turn and normal summoned Witch Raider or special summoned Anki, then flip summoned my tuner, I could not synchro summon this guy out. Wtf is the reason for this failure? The numbers add up, all materials are present and face up, yet just by flip summoning the tuner it won't work. Is this a bug or some rule I'm unaware of?
Read Witch Raider's text, particularly the final line.
If it's just Anki (no Witch Raider), the Synchro Summon should be able to happen, though. Neither Mask Change nor Anki itself limits your summon for the turn. So I can only assume the OP used the wrong Level for the Tuner, since to Synchro Summon the Level 9 Giganticastle using Level 8 Anki as non-Tuner, the Tuner can only be Level 1.
Thanks, Witch Raider was the culprit.
I played the Vagabond with the challenge that lets you normal summon twice per turn. Normal summoned Witch Raider, cleared his backrow, then normal summoned Changer Synchron and couldn't summon Giganticastle. Came back to check on this comment and then read Witch Raider's text. Yup.
How to heck do you summon this guy tho. Doesn't work while having 1 tuner and 1/2 non-tuners on the field