No special sense in the way you're thinking: it's a moderately rare word, but it's also a compound built out of two common words -- φερ- 'bear, carry', and ἀσπιδ- 'shield', so at the same time as being uncommon it's pretty unexceptional.

The fact that it's in the vocative case is pretty big giveway that it's quoted from a poem known as the 'Homeric' Hymn to Ares, line 2, which is one of only two instances in pre-modern Greek literature where it's used in that form. The Hymns are a collection of mostly very early poems on mythology surrounding the gods, which gives them a certain prestige. One of the shanties in Assassin's Creed Odyssey also used the text of this hymn.

Unfortunately for the song-writers who drew on this hymn in the games, the Hymn to Ares is an intruder in the collection: it was composed in the 5th century CE or later, well into Christian times (though the author wasn't a Christian). In the song you're asking about, the song-writer strangely mangles the word, putting the stress accent on the wrong syllable, and I find it hard to imagine that a speaker of modern Greek would make that mistake. So it's pretty transparent that its place in the song isn't something of deep significance, but a decorative gesture, borrowed from a poem about a Greek god who happens to be somewhat relevant.

On the date of the hymn: it's always been obvious that it's is out of place -- it's in a totally different style from the rest of the Hymns, it just lists off compound epithets for the first several lines, in much the same way that the song in the game is a list of ideas and qualities. A 5th century CE date (or later), specifically, is suggested by strong parallels in a hymn to Athena composed by the Neo-Platonic philosopher Proklos Diadochos. Where the Hymn to Ares has

ὀβριμόθυμε, φέρασπι, πολισσόε, χαλκοκορυστά

strong-hearted, shield-bearer, city-saviour, bronze-helmeted

Proklos' hymn to Athena, at line 3, has

ἀρσενόθυμε, φέρασπι, μεγασθενές, ὀβριμοπάτρη

male-hearted, shield-bearer, super-powerful, strong-fathered

This is certainly the model for the line in the Hymn to Ares. So in fact the use of 'shield-bearer' in the game originates with Athena, rather than Ares. The word φέρασπις does appear in a couple of classical-era texts too -- two of Aischylos' plays, the Persians and the Agamemnon -- but as I said, the use of the vocative form (albeit mispronounced) strongly points to the Hymn to Ares as the inspiration.