As you note, 特別【とくべつ】 followed by に would have to be an adverb. As such, "special battle" cannot be the correct interpretation of 特別【とくべつ】に戦【いくさ】: just grammatically, that would be a mistaken translation into English, and as a Japanese phrase, we know we're missing something.
So we look again at the fuller phrase, 特別【とくべつ】に戦【いくさ】が神【かみ】.
Considering the context, and that 神【かみ】 here is an adjective meaning "incredible, fantastic", we can now tell that 特別【とくべつ】に acts as a modifier on the adjective 神【かみ】, and not on the noun 戦【いくさ】.
Unpacking the meaning then, we have a 特別【とくべつ】に "especially" 神【かみ】"incredible, fantastic" 戦【いくさ】 "battle".
There's a nuance to this word order. The Japanese word order works out where the core phrase is the statement, 戦【いくさ】が神【かみ】 "the battle is incredible", and this whole statement is then modified by the 特別【とくべつ】に "especially / extraordinarily / particularly". This is a slightly different shade of meaning than if we'd said 戦【いくさ】が特別【とくべつ】に神【かみ】, which is straightforwardly "the battle is especially incredible". To preserve the nuance in English, we might use a similar word order, and translate this as "especially / extraordinarily / particularly, the battle is incredible".