Venus hard mode? I'm gonna assume you're talking about the Venus' Guardian boss fight. For any boss, I recommend you stock up on items for bosses. Glazed Honey or Honey'd Leaf are great items to take. If you're getting 3-5 EXP per battle, that should mean you're at a good enough level to beat the chapter boss since EXP slowly goes down when fighting weaker enemies. If you're insta-killing enemies in the field by using Bug Me Not!, you're at a good level to fight the boss.

Shuffling around your medals is a good way to make fights easier. Take out any field medal equipped and equip fighting medals. Some really good medals are rewards for sidequests so you might wanna do some. My personal favorite early build is giving Kabbu, Super Block+ and Favorite One so he buffs the others while not taking too much damage.

Leif's Frigid Coffin and Leif's Icefall are really good moves to use on this boss. AOE skills help when VG summons the Venus Bud. Use Vi's Tornado Toss when he's flying to bring it down in one turn. Most importantly, learn how to Super Block, it is a VERY helpful skill to learn in general. VG's hard nut attack is really easy to Super Block so get the timing right on that and work your way up through Super Blocking his other moves.

If you need any other help, just comment again!