When ending a sentence with an adverb, do I need to use copula?

Question

pretty basic question, but I was curious if an adverb needs copula (like a noun, or na-adjective) or not (like an i-adjective).

For example:

1. たいへんだそう or たいへんそう
2. ぴったり（+だ？）
3. 私はズバズバ（+だ？）<- can i even use a sentence like this?

Answer 1

In casual/colloquial speech, you can often (but not always) drop the copula だ・です after adverbs such as くたくた、ふらふら、こりごり、がらがら (同じ音を繰り返すもの), がっかり、しっとり、さっぱり (「〇っ〇り」「〇ん〇り」の形のもの) -- those that generally can function as a する-verb or an onomatopoeia.

e.g.

このぬいぐるみはふわふわだ。 → このぬいぐるみ、ふわふわ。
おなかがぺこぺこです。 → おなかぺこぺこ。
このスカートはぴったりだ。 → このスカート、ぴったり。
もううんざりです。 → もううんざり。

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たいへんだそう or たいへんそう

Actually, no adverb is used in these examples. The そう's used here are an auxiliary (dictionary form: そうだ), not an adverb. たいへん's used here are both a na-adjective, not an adverb.

But if you're asking whether they can be used at the end of a sentence with no copula attached...

(様態) たいへんそうです。 → たいへんそう。

It's common to use it that way in casual/colloquial speech.

Regarding:

(伝聞) たいへんだそうです。 → たいへんだそう。

It's also okay, though you might see 「～だそう。」 more in the written form (eg ブログやエッセイなど). (In casual/colloquial speech, we often say 「たいへんらしい。」「たいへんみたい。」 or 「たいへんなんだって。」 etc.)

Answer 2

I think especially in colloquial speech, you can omit copulas, but in rather formal speech, copulas such as です、ます would be requited. By the way, I don't understand what this

私はズバズバ

sentence mean. Regarding your 2 sentences, which are

たいへんそうだ

and

たいへんだそう

are both quite different, because whereas たいへんそうだ denotes the subjective view of the other's behavior, like "It looks like A is quite dreadful in XXXX", on the other hand, たいへんだそう denotes like "I heard A is in trouble", meaning a hearsay or a report from someone. Since these 2 sentences are quite different, I would like to refrain from speaking about copulas.