Warlocks start with light armour proficiency, if you don't want to wear it you can take the invocation Armour of Shadows at level 2 which gives free casting of Mage Armour so you don't waste a spell slot. Scrolls for this are fairly common as well, use these on a wizard to not waste spell slots. Cantrips are really overpowered compared to 5e and you never run out of these. Use Ray of Frost on an enemy standing in blood so they can't get to you as fast. Tactical positioning as well can keep you safe, also everyone can use the Hide bonus action when out of sight range. It is the common behaviour to take out the casters/healers first so it is good that the enemy also recognizes this.