Context-Free Grammar from this language
As Aspass.Jack wrote in the comments, let's first define $L_3$ to be the language where each $a$ is followed by at least 2 $b$'s, and $L_4$ to be the language where each $b$ is followed by at least one $a$.

Now, if let's say you can define CFGs for $L_3$ and $L_4$ with start symbols $S_3$ and $S_4$. Then, we simply need to define a grammer with the transition:
$$S \rightarrow S_3|S_4$$
together with all the transitions from the other 2 CFGs. Hopefully this simplifies your problem.