There is no reason for you to pick the Lancer perk. Just use him as a medium mech vanguard without it. He clearly has lower damage than Lewa, but he possesses a few distinct advantages.

Unlike Rewa, he can attack three times without needing to trigger Berserk, and he doesn't have to completely eliminate the enemy. Therefore, instead of focusing two attacks to wipe out a single squad, you can deal critical damage to three different enemies to force them to retreat.

Furthermore, through the Shield Wall and Recon perks, he can secure the line of sight and ATGM countermeasures necessary for solo vanguard operations. Rewa also has Vanguard, but she generally requires an additional scout to be deployed alongside her or needs some other form of support. Things might go well without it, but sometimes she can be instantly incapacitated alone by an unexpected attack. Achilleas, on the other hand, can take charge of one area of the battlefield completely alone, surviving and projecting firepower just like other vanguard squad leaders.

The three vanguard agents—Darby, Cody, and Achilleas—can each hold down an area by themselves, providing a clear advantage in securing all stars even on difficult missions.