Overview
Mk.18 is a buffing AR. Overall, she’s somewhat similar to a pure DPS buffing HG and is technically an option to put on position 2 in an ARSMG. She also gains some self-buffs for enemies being close to her and losing links, but these can largely be ignored since losing links is not advised and the buffs do not provide enough value.
You are far better off just using P22, who is permanently farmable, brings additional defensive utility, and has better-suited stats for position 2. Mk.18 is very optional at best, and more realistically not recommended.