Here is the full list of all 20 Behold Battle achievements.
Survive level 10 on normal difficulty
Survive level 15 on normal difficulty
Winner vs random: Survive level 20 on normal difficulty or harder.
Random vs random: Survive level 20 on normal difficulty or harder.
Combat: Survive level 20 on normal difficulty or harder.
Combat + Spells: Survive level 20 on normal difficulty or harder.
Small battle: Survive level 20 on normal difficulty or harder.
Medium battle: Survive level 20 on normal difficulty or harder.
Large battle: Survive level 20 on normal difficulty or harder.
Huge battle: Survive level 20 on normal difficulty or harder.
Gigantic battle: Survive level 20 on normal difficulty or harder.
Win 3 rounds in a row.
Survive level 20 on brutal difficulty or harder.
Survive level 20 with 500 gold or more.
Survive level 20 with 2000 gold or more.
Win 5 rounds in a row.
Survive level 20 with 5000 gold or more.
Win 8 rounds in a row.
Survive level 20 on insane difficulty or harder.
Survive level 20 on impossible difficulty.