Just one sense, and obviously best as an adjective.

@@ -14,0 +15 @@
+<pos>&adj-no;</pos>
@@ -16,7 +17 @@
-<gloss>long military service</gloss>
-<gloss>having experienced many battles</gloss>
-</sense>
-<sense>
-<pos>&adj-no;</pos>
-<gloss>seasoned (e.g. soldier)</gloss>
-<gloss>veteran</gloss>
+<gloss>veteran (e.g. soldier)</gloss>
@@ -23,0 +19,2 @@
+<gloss>seasoned</gloss>
+<gloss>(of) long military service</gloss>

歴戦	55192	  
歴戦の	50884
歴戦の勇者	7166	  
歴戦の勇士	5748	  
歴戦の傭兵	3385	  
歴戦の猛者	2902	  
歴戦の傭兵エキドナ	2724	  
歴戦の強者	1992	  
歴戦の戦士	1529	
... etc.

https://eow.alc.co.jp/search?q=歴戦の
歴戦の
battle-tested
単語帳
歴戦のを含む検索結果一覧
歴戦の兵士
experienced soldier
veteran《軍事》
歴戦の勇士
battle-scarred veteran
a brave and experienced soldier

Chiefly an adjective, so I think it should have adj. glosses available (again, not easily derived from the nouns).  The specific encounter was 
歴戦の老戦士
1. a seasoned old warrior
2. an old, seasoned warrior

that's +2 for "seasoned".  "veteran" is fairly common in reverso. I pulled battle-hardened from reverso, which I saw a couple of times. Maybe a bit strong, but eijiro give "battle-scarred veteran", so I don't know. Eijiro's "battle-tested" actually seems too weak.  Not enough implied battling....

〘名〙 何度もの戦争を経験したこと。何回も戦ったことのあること。 <- lots of battling

@@ -16 +15,0 @@
-<pos>&adj-no;</pos>
@@ -18,0 +18,6 @@
+</sense>
+<sense>
+<pos>&adj-no;</pos>
+<gloss>seasoned (e.g. soldier)</gloss>
+<gloss>veteran</gloss>
+<gloss>battle-hardened</gloss>

daijr: "何回も戦場で戦った経験があること。"

Specifically fighting.

@@ -17,0 +18 @@
+<gloss>having experienced many battles</gloss>