promote your archer to sniper, don't take the hit on agility and movement it will cost you in the end. I used Elric as a sniper my last playthrough and his attack was well above 100 because he was able to hit (and most of the time kill) enemies every turn because he had high agility and decent movement
A sniper gets that + 1 on every stat and the occasional +2 in 1, or seldom 2 random stats every level up whilst the brass tank makes sure the archer gets +2 on HP and Def and occasional +1 on others on every level up and has 1 move point less. A brass tank isn't all that bad as everyone say, but the deal of 1 move point for more, lots more Def and HP is just too much for someone who'll remain as a secondary wave in terms of offense(at the back lines) cause of average Agi and originally low Def and HP. You'd rather use those move increasing stuff on a others anyway.