Yeah, a cleric tank seems kind of iffy. You need damage, tanking doesn't really do anything in this game. Try to get a fire incarnate to kill the weaker dudes with fireball. Also tip: You can right click examine enemies to check their resistances. Sebille should have more damage in the form of backlash, rupture tendons, piercing stab, multiattack. She should be able to nearly kill a beetle on her first turn.
It seems you don't have any good elemental aoe, or physical cc+aoe, which are pretty necessary. A physical dude can have whirlwind, battle stomp, bouncing shield for lots of damage. Air is a bit weak early on and geo+pyro is really where the damage is(best early skills being impalement+fireball).
When you talk to her, have the rest of your team back off to the high ground where you come in from. Spread out a bit.
Buff(and try to get a fire incarnate before the fight starts), retreat the character talking(you can teleport a character in dialogue without starting a fight), start the fight, and let the enemies come to you. Focus the beetles.
The rest of the enemies just group up on the ramp and you can aoe them to death pretty quick(although it seems you don't have geo or pyro, so maybe melee aoe?). wait for your teleport to go off cooldown after retreating your character, and teleport one of the beetles down on the ramp to start the fight, or start with an attack on one of them.
