The phrase "more mettled" refers to someone who is particularly brave, determined or resolute. If you're looking for synonyms for this phrase, there are plenty of options that can help you to convey a similar meaning. Some possible synonyms include "courageous", "fearless", "intrepid", "valiant", "steadfast", "unflinching" and "unyielding". Depending on the context in which you're using the phrase, you may also consider words like "driven", "ambitious", "hardworking" and "dedicated". Ultimately, the right synonym for "more mettled" will depend on the specific qualities you're trying to emphasize, and the tone you're aiming to set with your writing.