Killing the man while he's stuck in prison is the simplest option, as long as Zeff is willing to reveal the time travel magic.
With the memory scroll trick, he apparently doesn't need to worry about being believed, so it's only a matter of willingness.
Since he already showed this to all his team, there is a strong possibility he'll now explain the situation at least to the five head mages.
From there, the plan to take Grain's life while he's vulnerable is definitely an option.
Otherwise, Zeff needs to find a way to strengthen himself roughly in the same way as Grain does.
Maybe he can get himself locked up in a mind prison in the same way.
Or use a time skip trick to repeat a limited time of training multiple times.