tough

adamant aggressive arduous awful bad brawny brawnier bully bullies burdensome callous chewy conscientious courageous crabbier crabbier crabby/crabbed crabbiest crabbiest crabby demanding difficult difficult dispassionate durable effortful efficient energetic exacting exigent feisty firm formidable forbidding gangster glutinous gritty grievous hard hard hard hardheaded harder harder harder hard-nosed hard-nosed/hardheaded harshest harsh harsher healthiest healthy heaviest heavy Herculean high-grade hood incompliant incompressible insensitive insusceptible intractable intimidator intractable/intransigent kinetic knockabout knotty laborious leathery lusty malevolent more intransigent more intractable more intractable more aggressive more hardnosed more hardnosed more efficient more hard-nosed more hard-nosed more difficult more difficult more hard nosed more hard nosed more durable more demanding more crabbed more hardheaded more hardheaded more thoroughgoing more stringent more robust more serious more primary more thorough most crabbed most awful most efficient most hardnosed most hardnosed most hard-nosed most hard-nosed most intractable most intractable most intransigent most hardheaded most hardheaded most hard nosed most hard nosed most primary most robust most serious most thoroughgoing most thorough mucilaginous muscular nasty naughty obdurate oppressive particular praiseworthy primary/prime primest prisoner prisoners puissant rascal remorseless reusable resilient robust rocky rugged rugged rough roughest rough-and-tumble roughneck rougher serious severe spunky stable stalwart staunchest staunch stern stiff stony stout stouter strenuous strict stricter strictest stringent stringy strong strong strong stubborn sturdy taxing tenacious thick-skinned thorny thorough/thoroughgoing thoroughpaced tighter tightest tight tiresome troublesome trying tyrannical unbendable uncompromising uncompassionate uncompliant unfeeling unhospitable unpitying unsavory unrelenting unstirred uphill unsympathetic unyielding vigorous vicious viscid viscose valorous viscous weighty wiry