You said you're already Barb 20, so none of the Druid stuff below applies.
At this point, as Daniel Zastoupil stated you just want to avoid falling unconscious.
Some things I would look in to to help would be:
- Get a Death Ward cast on you, either through a scroll or a party member or other powerful caster that you could pay for.
- Look for items or spells that stop you from being put to sleep through magical means.
- Get a Ring of Protection (Extra AC and help with saves).
- A Ring of Spell Turning could be helpful to stop those pesky mages from putting you to sleep or otherwise screwing with your brain.
- LOTS AND LOTS of health potions, to keep yourself well above 0HP.
- Have party members that can keep healing you or bring you back if you do go out.
Moon Circle Druid multiclassed over into Barbarian makes for a really tanky combo
This part doesn't directly apply to Ivan, but this Druid/Barbarian combo makes for a really strong build that doesn't ever hit zero HP in the first place.
To start, let's assume Druid 15, Barb 5, and 20 CON from level 1. Yes, maybe slightly unlikely but we're talking about an optimized build here.
Time for some powergaming
At level 15, a Moon Druid can Wild Shape into a Triceratops twice per rest. The Triceratops has 95HP, times two is 190HP. Since we assumed 20 CON, if we assume that you take average HP every level, you have 15*(5+5) + 5*(7+5) for a total of 210HP on your main character in his true form. Adding these up, that's 400HP between your Wild Shapes and your true form.
Furthermore, Moon Druids can expend one spell slot per turn and regain Xd8 HP, where X is the level of slot expended. At level 15, the Druid will have 62d8 of healing available, which averages 279 HP of healing. Adding this into our running total, we get 679 HP!
Barbarian adds some spicy features
We multiclassed into Barbarian for the Rage feature, which carries over into Wild Shape AND can be activated while in Wild Shape if it goes down. Now, here's where things get crazy.
If you take the Totem Warrior Path at Barbarian level 3 and select the Bear Spirit, your Rage gives you resistance to ALL DAMAGE that isn't marked as psychic. This includes magic damage of any sort. If it doesn't say Psychic, you take half. This means that, if you keep your Rage up for the entire fight, you have an effective HP of 1358!!! (Barring any Psychic damage).
But wait...there's more!
Barbarian also gives you Unarmored Defense, which means that if your Wild Shape AC happens to have less than
10 + DEX mod + CON mod, then your Wild Shape's AC becomes that number. With +5 to CON, this feature will pretty much always be active.
You also get Reckless Attack, which can be useful if you want to help chunk somebody down. At level 20 most things are probably hitting you anyways, so advantage against you only increases the odds of crit. You also get Danger Sense which can help against save-or-die traps.
At Barb 5, you get Extra Attack (self-explanatory) and Fast Movement, both of which carry over into your Wild Shape, so you can hit twice and move 10 feet more per round. With Extra Attack on the Triceratops, if both attacks hit you can be dealing 8d8+12 damage each round, or an average of 48 damage. DPS classes will pass you by a mile but remember, YOU'RE THE TANK!
Final Note: It's your choice whether you go for Druid15/Barb5 or Druid17/Barb3 for the extra Druid spell slots (which directly translate into healing) in exchange for the Extra Attack and the Fast Movement of the Barbarian.
Druid/Barbarian makes a pretty broken combo, but for Ivan you want to look into Death Ward, Ring of Protection/Spell Turning, lots of health potions, items or spells to prevent magical sleep, and lastly and most importantly, a helpful party. You'll never tank the universe on your own, but with the help of a well-optimized level 20 party you can take down pretty much the entire Monster Manual.