この問題は複雑ですが厳しいじゃありません

[dpaint4]

この問題は複雑ですが厳しいじゃありません。あなたはすごいい学生です。がんばってね！

Here's the Hiragana version:
このもんだいはふくざつですがきびしいじゃありません。あなたはすごいいがくせいです。がんばってね！

Do I sound like an idiot? I synthesized that from phrases I've learned from my book and from the podcast so far, but I'm sure the connotations are all wrong, as well as the grammar. I'd love for someone to correct me.

[Bueller_007]

この問題は複雑ですが厳しいじゃありません。あなたはすごいい学生です。がんばってね！

Here's the Hiragana version:
このもんだいはふくざつですがきびしいじゃありません。あなたはすごいいがくせいです。がんばってね！

Do I sound like an idiot? I synthesized that from phrases I've learned from my book and from the podcast so far, but I'm sure the connotations are all wrong, as well as the grammar. I'd love for someone to correct me.

この問題は複雑ですが厳しいじゃありません。あなたはすごいい学生です。がんばってね！

Two things:

1) 厳しい is an -i adjective, so you've constructed the negative wrong:

-na adjectives:
remove the "-na", add "de[wa] arimasen" (or "de[wa] nai desu")
無理ではないです

-i adjectives
remove the "-i", add "ku[wa] arimasen" (or "ku[wa] nai desu")
厳しくないです

2) The word "sugoi" (凄い) only has one "i" at the end.

Otherwise, the sentence is fine, and I think you'd have no problems being understood, even as it is now.

Personally, I wouldn't have said 厳しい, I would have said 無理. And you generally should avoid あなた and just use the person's name, or omit it altogether and let the context make it clear. Also, beware the difference between 学生 and 生徒. The word 学生 primarily applies to university students. 生徒 primarily applies to students in middle or high school. It's also the word generally used to describe students in private schools like juku or eikaiwa. So that's probably a better choice for you. That or maybe the word スチューデント.

Casually, you might try this:
この問題は複雑だが、無理だというわけじゃない。才能豊かな生徒なんで、がんばったらよくできるだろう。

[dpaint4]

You're amazing. Thanks a lot. I'm going to print your reply and add it to by notebook.

[dpaint4]

この問題は複雑ですが厳しくないです。すごいスチューデソトです。がんばってね！

Okay. I've fixed all the things you marked and I'm going to drill this for a while. I need to learn the word you suggested to replace 厳しい. I don't currently know that term, so I can't use it without just blindly using it. But I'm going to learn it tonight.

[Bueller_007]

この問題は複雑ですが厳しくないです。すごいスチューデソトです。がんばってね！

Okay. I've fixed all the things you marked and I'm going to drill this for a while. I need to learn the word you suggested to replace 厳しい. I don't currently know that term, so I can't use it without just blindly using it. But I'm going to learn it tonight.

You HAVE to know 無理 (むり). That's one of the big ones.

It means "unreasonable", "illogical", "[almost] impossible", or "to overdo something".

Remember it well.