You don't necessary have to melee them in order to deal damage, explosive guns should be fine too. Furthermore there are many ways to avoid "laggers" in both Complex and Multiplex such as the ping limit rooms.

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Also you can simply report them, in most cases staff will relog them which of course does not solve the issue of people abusing their wifi, but nonetheless it will be a temporary fix since most people (including me too) actually leave the spar arena after being relogged