Labyrinth+D Gate should be fine for both levels. Guardian? You mean Elements Unite? It's the worst skill against him. What are you going to do against his Crass Clown? Even if you run Grarl, how are you going to make sure you draw it on your 2nd turn?
you dont lose anything by attacking bonz' level 30 monsters, so crass clown can only threaten gate guardian if bonz uses call of the mummy and summons two clowns in one turn. that said, skull invitation massively fucks up elements unite farming.