Remember prime Mercer was impossible to blow away (see Lewis) and Foreman was not a great boxer. Who wins?
Foreman wouldn't blow him away but with his huge strength and advantage in size he would keep Mercer off and pound a clear win in a bruising fight
Foreman would stop Mercer on his feet in about 7 rounds. Doesn't matter how good your chin is, when you take that many punches against a guy like a prime Foreman, you're in a world of hurt.
He went toe 2 toe with LL, who may not punch quite as Foreman, but was more fluent & accurate, & was never really rocked.
Mercer had a great workrate & stamina, & was only ever out-boxed. Foremans stamina was not great. Peralta gave him a tough fight.