What the achievement asks
Reach Renown 25 with the Maruuk Centaur. Five points, account-wide, both factions.
What renown actually costs
Renown with a Dragonflight faction comes from almost everything you do in its zone: world quests, dailies and weeklies, rares, events, and the faction's own questlines. There is no single fast route and nothing you can buy to skip it.
What there is, is a floor. Renown never resets and it is account-wide, so every character you play feeds the same bar, and a faction you start late pays more per activity than one you have been grinding since launch. Coming to this now is considerably faster than it was at the time.
Do the weekly Aiding the Accord every week without fail. It asks you for 4,000 reputation from whatever you were doing anyway, and it pays out a chest that gives 500 renown reputation to every main faction at once. Nothing else in the expansion moves six bars with one turn-in.
The Grand Hunts do the work
Maruuk Centaur renown comes largely from the Grand Hunts in the Ohn'ahran Plains, the Waking Shores and Thaldraszus, and from the zone's world quests.
That is convenient, because three separate achievements already want you in those hunts: Longhunter for a hundred hunt steps, The Best at What I Do for thirty final bosses, and Tetrachromancer for nine companion colours.
Thirty hunts for the bosses is more than enough hunting to carry this renown a long way. Run them with all four objectives in mind rather than treating the renown as a separate grind, and it arrives on its own.
Players who reached Renown 25 doing nothing but Grand Hunts report still being short on the companion colours, which tells you which of the two is the real bottleneck: it is not this one.
Where it fits
One of twelve under Oh My God, They Were Clutchmates, a piece of A World Awoken. Renown is the slowest currency in the meta and the one with the least to do with skill, so start every faction early and let the weeklies carry them.