She's using updated variants of old ideas because they don't seem out of place in this universe. They're based on modern weaponry(Such as mortars and the like), but they're been technologically upgraded so they don't suck like our stuff is, at least comparative to this universe. Think of the explosives she's now using as like the Krak missiles or Krak grenades the Imperial Guard uses to deal with heavy infantry. 

They're not bolter tier weapons, but the explosives should do a better job at being more than a minor annoyance like the machine gun. She had originally came on this mission expecting just some soldiers with laser guns, not a walking super-tank like your Termie. If she had known he was here or had any idea of his capabilities before hand, she'd be using plasma weaponry or very high tier weapons. He's an experienced and powerful opponent(She'll prolly respect him as an enemy after this), so she isn't taking him lightly, and has escalated her response to using anti tank weapons accordingly. She doesn't like to use higher tier stuff because it tends to leave more of a footprint then she likes(It attracts attention and leads to situations like the one Lina is in currently), and revealing her full capabilities too early allows her opponents to counter her skills, which is something she also dislikes.

[@Snarfulblast]
I love the Necrons. If you have any questions I'd be glad to help.