I've seen both pronunciations (at the beginning of the second bracha in shmoneh esrei):
- Aleph-patach, tav-kuhmuhtz, heh (implied stress on second syllable)
- Aleph-kuhmuhtz-meseg, tav-kuhmuhtz, heh (noted stress on first syllable)
It should be מלעיל, because of the rule of נסוג אחור
Since the next word (רב) has its stress on the first syllable, the stress of the preceding word moves up to avoid having two stresses in a row.
However, if רב להושיע is a separate phrase, this rule might not apply.
I don't if it's an official rule but I remarked (in Sepahradi Siddur like Ich Matsliah') that when Ata is after the attribute, the word is mileel (stress on the second to last syllable) but when the word is before the attribute, it's milera (stress on the last syllable)
Ex: At*a* guibor Ex: Melekh Rah'oum Veh'anoun *A*ta.