"toughest phase" is a correct and usable phrase in written English.
It refers to a difficult or challenging period or stage in someone's life or a project. Example: After the sudden loss of her husband, Sarah went through the toughest phase of her life, struggling to cope with her grief and adjust to being a single mother.
Cook argues that scrutiny can only get harder for Trump, while Clinton may have been through the toughest phase.
The next few days will be defining for Adam Yates as the 23-year-old from Bury goes into the toughest phase of the Tour de France holding third place overall behind Chris Froome in the yellow jersey.
Addressing a packed parliament before the vote, Merkel urged the lower house Bundestag to fulfill its "historic duty" and said Europe was in its toughest phase since the end of World War Two.
(Female participant) The midwives and nurses were the best emotional supports for me during the entire time because I was going through the toughest phase of my life.
The midwives and nurses were the best emotional supports for me during the entire time because I was going through the toughest phase of my life.
Athleta is now in one of the toughest phases for a small business: becoming big.
Second, there has to be a "get tough" phase.
The tougher phase of standards (beginning next year) calls not just for downsizing but for new cars.
It was a tough phase, she recalled, but "Husband No. 3 is what set me free".
Then, 90 days after the well is finally plugged, comes the tougher phase of the three-year program: negotiating with each claimant for the lump sum to cover economic losses from the spill.
The missile strike represented a tougher phase of the campaign against terror and moved the Bush administration away from the law enforcement-based tactics of arrests and detentions of Qaeda suspects that it had employed outside Afghanistan in the months since the fighting there ended.
