a bit (adjective)

"A bit" is similar to "a little", "kind of" or "somewhat". It's used when you want to lessen the degree of a description:

He's a bit shorter than me.

He has a video of me from when we were in college that's a bit embarrassing.

"A bit" is used for adjectives that are negative. So you wouldn't say that an event was "a bit fun" or that a person is "a bit nice". But you can say that the event was "a bit boring" and the person is "a bit rude".

"A bit" is slightly formal. It's more formal than "kind of" or "a little", but more casual than "somewhat":

kind of > a little > a bit > somewhat

(someone) can be (adjective)

Use this phrase to describe a negative characteristic that a person sometimes has:

She can be really hard to work with sometimes.

He can be painfully shy when he's around people he doesn't know, but to those of us who know him he's actually really talkative.

Saying that someone "can be ___" is a good way to say something negative about a person, while still appearing to speak positively about them. So you might use this carefully to complain about your boss with a coworker, for example.

It's common to use "sometimes", "at times", or "from time to time" along with this phrase.

(someone) is assertive

An "assertive" person is someone who strongly states and fights for their own opinions. A good image for the word "assertive" is a person who is able to ask their boss for a raise when they think they deserve it.

The word "assertive" has a positive connotation. You can often hear people telling their children or spouses:

You should be more assertive!

But it's not strongly positive. People want to have this quality for themselves but might find it a little annoying when other people display it. So in the example above, the speaker is able to use this as an example of a "weakness".