Sticking with an adjective borrowed from Latin, in which language maior (= 'major') is a comparative from the root magnus, the English adjective formed from the borrowed superlative of the same root is
1. Of or constituting a maximum; of the greatest possible size, duration, or capacity.
["maximal, adj.". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/115258 (accessed December 20, 2015).]
In your example sentence, this might be phrased as
After fighting through several minor encounters, and a few major encounters, the adventurers finally fought through the maximal encounter.
Because 'minor' and 'major' are adjectives in English (regardless of being borrowed from comparatives in Middle French and Latin, respectively), the superlative adjectival phrase
is another possibility. This would be a rare use, but not unprecedented. Hence,
Their fighting ranged from the most minor to the most major encounters.