The Western Xia were roughly similarly powerful than the Western Wei in the 6th century, and the latter then went expanding as the Northern Zhou which then was the building block of the Sui dynasty.
The OP specifically required a wank - I don`t think Western Xia conquest of China is the likelist of all options, but if Western Xia went on a conquering spree, I´m sure it wouldn`t go steppe-based. It would go mountain-based, if the place they lived in and their warfare was adapted to tells us anything.
Their ties were closest with Tibet, so if we want to wank Western Xia, I suppose formal vassalage and massive recruitment of Tibetans could be a start. Controlling the Tarim Basin and thus the Silk Road is the likeliest first target. Wrestling the control over the Eastern Chinese-Indian trade route from the Dali could be the next step. Controlling all that commerce makes for good funding of any campaigns, and the next ones would harass the Song. While a steamroller conquest is certainly implausible, effective blows are not beyond imagination, while the Western Xia themselves are relatively difficult to counter-attack, sitting as they are in the mountains. Maybe Song becomes so destabilised that it fractures into several smaller entities and sinks into civil warfare? In that case, a peacemeal Xia conquest from the West could be within the realm of the possible. Once half of that is achieved, the Liao in the North are ripe for the plucking. The last steps of unifying conquests always seem to happen quick in China once the balance of power has begun to tip.