18/24 with the Mallet (or 15/27 with the regular Maul) is pretty hilarious; the damage you get from speed bonus results in a ton of knockdowns, and like any mauler build, it's all about patience and good blocking, waiting for an opening to attack.
The most common route, especially for Siege mode oriented builds, is high strength with the Great Maul, so something like 30/12 or 33/9 (Ath, PS, IF or WM with leftovers), or a more fleshed out 30/9 build (max WM, PS, ATH, 7 IF). You can also afford to make more mistakes with these builds due to HP and armor (presumably).
21/21, however, can be quite surprising to enemies. It still hits very hard with 7 PS (because you're using a goddamn Great Maul), but you can choose your fights and position better with 7 Athletics. The downside is you'd have to sacrifice some health, as 21/21 will only leave you with 3 extra points after getting Athletics and PS.