If you’re happy to wear armour, put on some armour and pretend you’re not a wizard in battles. If you’re not, you will want to pick up something useful to do from behind the front line (heal, mend, purify, restore limb).
If you want a pure battlefield self-defense spell then Repel is where it’s at, but you probably want to have something useful to do on the battlefield first - that could be e.g. ‘I carry water’, ‘I run messages’ or ‘I hold a banner which shows people where the healing rally point is’ rather than necessarily something mechanical, though.
Remember everyone can use a buckler - unless your ‘what to do on the battlefield’ plan required your offhand to be in use at all times, it’s better than nothing as a second thing to hold.