Welder is the most basic ARM T2 general purpose infantry unit equipped with a lightning rifle. For a reasonable cost you get a bot with a decent armor and high damage output. The lightning cannon on the Welder can spread to 2 (nearby) enemy units, dealing 33% bonus damage.
Its disadvantages are low speed and short range, however, in combination with Sharpshooters (Snipers) they make up for a very powerful pushing force.
Always keep them in a line in front of the snipers, protecting them from fast unit swarms.