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In Spanish, we avoid using the -mente ending more than once.
So, your first sentence should be "Las tropas avanzaban lenta y cautelosamente."
To describe the troops themselves, instead of describing how they marched, you could use the adjectives in question (and they would most likely precede the verb).
The second sentence is technically not correct, and should contain the adverb you mentioned. But many people use the adjective when it shouldn't be used.
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