Он долго целился в мишень.

Целился is:

• past tense
• masculine singular
• from the verb целиться

In Russian past tense, the verb agrees with the subject in gender and number:

• он целился = masculine
• она целилась = feminine
• оно целилось = neuter
• они целились = plural

So целился matches он.

The verb is целиться, not just целить.

Here, -ся is part of the normal dictionary form of the verb and does not mean a literal reflexive action like he aimed himself. Russian has many verbs with -ся that function as standard verbs learners simply need to memorize as a whole.

So it is best to learn:

• целиться в кого? / во что? = to aim at someone / something

Because the verb целиться normally takes в + accusative when you say what someone is aiming at.

So:

• целиться в мишень
• целиться в цель
• целиться в птицу

This is just the standard pattern for this verb.

Мишень is in the accusative singular.

It looks the same as the nominative because мишень is an inanimate feminine noun, and with this type of noun the accusative singular is often identical to the nominative.

So:

• nominative: мишень
• accusative: мишень

Even though the form does not change, the function does: here it is the object of в after целиться.

Долго is an adverb meaning for a long time.

It tells you about the duration of the action. The idea is that he spent quite a while aiming.

Compare:

• Он целился. = He was aiming.
• Он долго целился. = He was aiming for a long time.

Russian uses the imperfective here because the sentence focuses on the process or duration of aiming, not on its completion.

That fits very naturally with долго.

• целился = was aiming, kept aiming
• прицелился = took aim, got himself aimed

So Он долго целился в мишень emphasizes that the action lasted a while. It does not say whether he finally fired, hit the target, or even finished aiming.

Yes, very often.

Russian frequently omits subject pronouns when the subject is clear from context. So in a suitable context, you could simply say:

• Долго целился в мишень.

That would still be understood as He was aiming at the target for a long time, if the context already makes the subject obvious.

No, Russian word order is fairly flexible.

Он долго целился в мишень is a neutral, natural order. But other orders are possible if you want a different emphasis, for example:

• Долго он целился в мишень.
  Emphasis on for a long time
• В мишень он долго целился.
  Emphasis on the target

The basic meaning stays similar, but the focus shifts.

The main stresses are:

• Он дОлго цЕлился в мишЕнь.

So:

• дОлго
• цЕлился
• мишЕнь

A learner may want to pay special attention to цЕлился, because the stress is on the first syllable.

No. By itself, it only says that he spent a long time aiming at the target.

It does not tell us:

• whether he fired
• whether he hit the target
• whether he finished aiming successfully

Russian, like English, can leave those details unstated unless another sentence adds them.