I don't think ice would be that effective, especially if the guy could increase the range of his shield, and if the plan was to drown him in water it would depend on who's magic power lasts longer, and also if other members wouldn't interfere which is something they wouldn't do. but earth magic would be more effective if she made a huge pit under him
I believe ice could be almost as, if not more effective than earth, say, for instance, their abilities couldn't be used to affect the terrain of this place. Like if for some unknown reason earth couldn't be used to create a pit because magic doesn't affect the floor.
Even if we grant this guy the ability to expand his shield to any range at will, it's doubtful that it could mitigate temperature. In that case, she could use ice magic to lower (or raise with fire, which I didn't mention before because it carries a much higher risk) the temperature around their pursuers to extreme levels that would cause their bodies to simply freeze (or melt).
Or ice could be used like earth to create large blocks that effectively seal the path behind the MC's group as they travel, buying themselves much-needed time.
Also, containing their pursuers within earth, (risky fire), ice, or water is another plausible option even if only to deprive their pursuers of oxygen. Similarly granting the kid could expand his shield, so long as it didn't exceed the range of magic they were covered with, they'd eventually run out. Likely sooner than his and her magic would if they were locked in a magic power struggle of sorts and especially so if he was trying to protect his classmates with shields as well.
Water I'd say is probably the most lethal option, even without trying to drown them outright. A well-placed drop of water used from a blindspot (like anywhere around the entrance to that room) could've been a one-shot without any of them ever knowing how they died.
I mean, most people won't even notice a loogie or snot-rocket on a dilapidated floor or wall where they're not looking or can't see. Fashion that into a needle, launch it, and it's all over. Or imagine stepping on a drop of water that pierces your foot, enters your bloodstream, and scrambles your heart or brain.
And these are only a few examples of how she alone could stomp these clowns. Add the rest of the MC's group, and it's an absolute joke.