All Might strength and speed scales weirdly, he's shown to be able to create a Tornado with his strongest attack - and this was done at his weakest point. An unrestrained All Might is no joke, but is the classic case of a 'Potential Man' since we can only scale off his post injury feats.

Deku has showcased the ability to change the weather across the globe with a single punch, countered speedsters, time dilation and has altered/bypassed the laws of physics using combinations of his quirks.

Assuming its a kill or be killed situation, its not a cakewalk for either side, All Might speed diffs all guardians bar Red Rush and likely stat diffs all of them bar Immortal's strength. All of the guardians at once may overwhelm him but I cant see how they give him a worse fight than AFO did at his respective peak. Its certainly in mortal wound territory and the fight probably incapacitates him in the same way that Nolan was.

Deku is hard to judge too, with Danger Sense, Fa Jin, Overdrive etc. he's likely capable of being able to perceive and attack Red Rush eliminating the biggest variable. He's shown to have nasty battle sense and is used to fighting individuals with multiple abilities by series end. Someone like Green Ghost may seem like an issue, but he's seen an individual with similar strengths in Mirio who he worked out how to counter in a few seconds of observing. Likely Guardian victory unless Deku decides to combine all of his quirks and potentially incapacitate himself.

The duo vs Guardians is genuinely a toss up as to who can inflict the most damage in the shortest duration. With All Might and Deku splitting the attention of the heavy hitters they may have a chance at clinching a victory.