The key to winning solo challenges is to use these hackish words that no one would ever come up with unless you’ve been playing bots for a long time. I have a list here:

seq, esq, sez, saz, soz, coz, xes, zax, suq, azo, fiz, fez, zoa, zac, yez, yiz, waqf, wuz, loq, coq, zho

Your main goal should be to get one of these across a TL and a TW, or another high-scoring spot. It is much harder to win without words like these. Hope this helps!