[The Saucepan Man]

From the History of Galadriel and Celeborn in Unfinished Tales:

When Sauron learned of the repentance and revolt of Celebrimbor his disguise fell and his wrath was revealed; and gathering a great force he moved over Calenardhon (Rohan) to the invasion of Eriador in the year 1695. When news of this reached Gil-Galad he sent out a force under Elrond Half-elven; but Elrond had far to go, and Sauron turned north and made at once for Eregion. The scouts and vanguard of Sauron's host were already approaching when Celeborn made a sortie and drove them back; but though he was able to join his force to that of Elrond they could not return to Eregion, for Sauron's host was far greater than theirs, great enough both to hold them off and closely to invest Eregion.

Then:

In black anger he turned back to battle; and bearing as a banner Celebrimbor's body hung on a pole, shot through with Orc-arrows, he turned upon the forces of Elrond. Elrond had gathered such few of the Elves of Eregion as had escaped, but he had no force to withstand the onset.

Later:

But his force was weakened by the necessity of leaving a strong detachment to contain Elrond and prevent him coming down on his rear.

It was during this campaign that Elrond, while surrounded by Sauron’s forces, established Rivendell as a refuge for the survivors of Sauron’s occupation of Eregion.

So Elrond was clearly a military commander during the Second Age. At some point thereafter, he renounced combat to concentrate on healing. The first reference to his healing powers that I am aware of was when he healed his wife following her abduction by Orcs (in TA 2509). That was well after the Last Alliance, so it is quite possible that Elrond took and active part in the campaign as a combatant. His designation as Gil-Galad’s herald would not be incompatible with this, in my view.