This route is often referred to as the normal route.
The common option is to follow the Southwest ridge bellow the rotten rocky crest. In this case the route is about Trekking with some easy scrambling (American Class 3 max / UIAA I ??).
However you may want to scramble the ridge itself. In this case you go for an Alpine climb with a theoretical grade of PD. Giving a rock climbing grade would be dangerous in this case: Beware that everything is badly rotten up there. It's not that much about technical difficulty that it's about progressing in a very unstable rocky environment...
The El Salto de Agua camp at 4200m is the usual start point for this route. It's situated 9 km & +1220m West of Vallecitos (7h30), next to a waterfall at the entrance of a "cirque" formed by (clockwise) Cerro Lomas Amarillas, Portezuelo Lomas Amarillas, Portezuelo Vallecitos, Cerro Vallecitos and Cerro Rincon.
From El Salto de Agua camp follow the trail south-west to La Hollada (also spelled Hoyada or Ollada) at 4500m / 4520m (45 min – 1h with light packs). Continue the trail in the same direction to the ridge. The trail reaches the ridge at a saddle (30 min – 45 min). This is Portezuelo Lomas Amarillas at about 4700m. From the saddle follow the ridge to the left (northeast). Because of the poor quality of the rocks constituting the ridge the common route option is to follow bellow, on the left side, on the scree. Later, high on the mountain (at about 5000m), you’ll have to cross a rocky “rib” that goes down from the ridge to La Hollada. There are few easy options available (American class 3 max). Once the rib crossed, you can head-up and reach the ridge itself and follow it to the summit as there is no major technical difficulty on the ridge in this area. From Portezuelo Lomas Amarillas to the summit count 2h30 - 3h or more due to route finding difficulties.
The gear one takes here is function of the route option (bellow or on the ridge) and the climbing season, but one thing is absolutely a must, especially if you climb with partners: a helmet. Do not underestimate the danger of stones following from the ridge.