I only see Hiruzen restrained on this pic. Where do you see Minato restrained? :| Furthermore what's the relationship between will and power level?Minato above Hashirama? :|.
Weren't Minato and Hiruzen both restrained even before Tobirama was? (note the *twitch* *jerk* actions by them, which were the same things that were shown when Tobirama was restrained):
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From what I know, it took the amplification of Edo Tensei's restraining properties through Hashirama's hacked cells to stop Tobirama.