Dwarves versus Necropolis.

The Fortress can easily withstand the Gnomes; their defense and HP are given for a reason, and they shouldn't attack. The Necropolis also focuses more on defense, but usually has a larger number of units. However, remember that two defensive factions usually prolong the battles, which is not what we need. The Necromancer will gain an advantage through upgrades; since he's already at level 31, he can take Spell Power + Luck, which will be very useful for the Gnome. There are creatures with greater defense here, and there's also magic and protection, so the Necropolis won't be able to break through, especially with the runes.