The dark mages of that time could be antagonists. They might sense Shirou's different magical energy and try to either control him or destroy him. Also, there could be traitors within Arthoria's court who don't like the idea of an outsider like Shirou being around and causing changes, so they work against him.
One idea could be Shirou trying to learn the ways of chivalry from Arthoria. He might struggle to adapt to the different moral and ethical codes of that time. Another could be Shirou using his modern knowledge to help Arthoria in battles, like devising new strategies.
A common caricature of a dentist might show them with a magnifying glass to examine teeth, and a bunch of shiny drills and pliers around them. The expression could be either serious and focused or comically panicked depending on the tone you want.
The characteristics of a cartoon-looking alien often include bright, out-of-this-world colors. They might have a friendly or mischievous expression and some kind of special power or gadget. Their design is all about catching the viewer's attention and being fun and memorable.
Honestly, 'cartoon ahwooga love eyes' is quite a strange phrase. It could potentially refer to a unique style of cartoon eyes that convey love, but without more context it's hard to say for sure.
One way is to look up unfamiliar English words in the manga and make notes. Also, try discussing the manga with English speakers to practice speaking. Another option is to write summaries or reviews in English about the manga you read to enhance your writing skills.