When operational, the Â£10bn nuclear plant is expected to create 850 permanent jobs and should start generating power by 2025.
But in a draft letter, the council has criticised Horizon for the lack of detail in its final consultation.
Horizon said it was "surprised and disappointed" by the council's response.
In the letter, the council's chief executive expressed concern at a decrease in proposed local employment during the building phase.
Dr Gwynne Jones also said there was "an objectionable lack of detail" on proposals to house 4,000 building workers on site.
The letter called for "immediate" investment in education to ensure that local young people will be employed at the proposed power station.
It suggested that, due to the lack of detail, the legality of the process could be in doubt.
"The council remains committed to working with Horizon and other key stakeholders but... not at any cost nor with promises of 'jam tomorrow'," the letter added.
A third and final consultation into the power station plans has been held and Horizon hopes to submit a planning application, known as a development consent order, later this year.
Responding to the council's comment, Horizon's Richard Foxhall said: "They have been made aware of what we intended to consult on before May 2017. They agreed with the content of what we do in the consultation.
Of course, what we've consulted on is what to change since last year, not necessarily on the whole project.
"However we do look forward to working with the council. This isn't the end of the game as far as input."
He added there was no question of the company's consultation process being illegal.

A Guardian article claims Al Hasawi, who has said he is close to selling the club, wants wages that could rise to £1m a year, as well as bonuses.
A United States consortium is on the verge of a £50m deal, which would see Al Hasawi retain a 20% stake.
Al Hasawi said "the information published was inaccurate".
"The terms of any potential investment deal are, and will remain, confidential and the club will not be making any comment in relation to this issue," a statement on the club website continued.
"The matter has now been passed to my legal team, as such it would not be appropriate to comment further at this time."
The takeover would see millionaire John Jay Moores, the former San Diego Padres owner, take an 80% stake in Forest.

The Downham Market blaze is believed to have started during the night where a £250,000 fire engine was parked.
The fire station, built in 2006, had no sprinkler system.
Norfolk fire service is still waiting for a full forensic study and said the inquiry was "long and complex" but an independent report had been received.
Cambridgeshire Fire and Rescue Service is assisting with the investigation and their findings reveal it is believed the cause of the fire was due to an electrical fault in one of the appliances at the fire station.
"We are in discussion with Norfolk Constabulary to consider options for a joint location at the site with shared accommodation," a spokesman said.
"We currently have temporary accommodation in place and the station is fully operational from that location."
At the outset the fire service recognised the investigation was going to be a "challenge" for experts.
The fire engine was so damaged and the heat was so intense a forensic investigation was needed but experts were optimistic they would find evidence of a cause.
Head of community safety Stuart Horth said the fire had created concerns.
"Checks have been made at other fire stations to identify potential risks and all were registered as safe," he said.
The £1m single storey fire station  was officially opened in July 2006.
It had an appliance bay where the fire is believed to have started, lecture room, offices, a kitchen and storage areas.
A year before the opening the fire station received a new state-of-the-art rescue pump.
The site was also shared with Norfolk Police and this was said at the time to be a great advantage.
The company behind Wylfa Newydd has been told Anglesey council will not support the scheme "at any cost".
[NEXT_CONCEPT]
Nottingham Forest owner Fawaz Al Hasawi has denied reports he is demanding a large annual salary and huge bonuses as part of any takeover deal.
[NEXT_CONCEPT]
An electrical fault in an appliance parked at a "state-of-the-art" Norfolk fire station caused a disastrous blaze in April last year, a report has said.