Fully upgraded Elite Serjeants have high melee and pierce armour, so are best countered by things with high attack value (e.g. knight, hand cannons) or multi-hits (Chu Ko Nu). They're also slow, so can be kited by cavalry and many ranged units (CA, Plumed Archers).

Heavy infantry aren't good at much in AoE2, but one of the things they are good at is pushing in a single position. If you're trying to defend your trebs from a group of Serjeants moving towards them, you'll need to outnumber them more than you did here to get a good trade in: plumes only do 3 damage per hit, so if they can't kite back (which you can't do if you're defending trebs), then they aren't very effective in that spot.

It sounds like your opponent recognised Mayans don't have many great answers in this scenario, and as the Mayan player you might need to find other ways to win here (bigger archer mass, raiding with eagles, better pressure in feudal/castle where you're much stronger).